A sixth goal of the season from Kanu combined with Andy
Cole's first-ever strike for Pompey extended West Ham
United's win-less streak to seven games following a miserable
day on the south coast.
And this fourth successive Premiership defeat now leaves West Ham in 18th place ahead of a tricky trip to Tottenham Hotspur next Sunday.
Both Alan Pardew and former Upton Park boss, Harry Redknapp, made a trio of changes in a bid to end their sides' losing sequences.
Pards welcomed back fit-again Anton Ferdinand in place of Christian Dailly, while ex-Pompey striker Teddy Sheringham made a rare start against his old club alongside Bobby Zamora, in place of substitute Carlton Cole and the injured Carlos Tevez.
Following back to back defeats against Bolton Wanderers and Tottenham Hotspur, Redknapp recalled Manuel Fernandes, Matthew Taylor and Benjani in place of Croatian Niko Kranjcar, Sean Davis and Lomana LuaLua.
Certainly, both teams seemed intent on changing their fortunes in the early end to end - albeit niggly - exchanges, as ex-Hammer Glen Johnson fired an early shot behind, before Paul Konchesky sent a 20-yarder well wide and Pedro Mendes sent a trademark 30-yarder whistling inches wide of Roy Carroll's right-hand angle.
Both Gary O'Neil and Nigel Reo-Coker were then booked by an over-officious Graham Poll for petty fouls, before Portsmouth finally broke the deadlock midway through the half, when Kanu got between Danny Gabbidon and Ferdinand to meet Johnson's hanging right-wing cross with a stooping, far post header that eluded the tumbling Carroll.
And with the famous Pompey chimes ringing out in celebration of the Nigerian's sixth goal of the campaign, Kanu should have had a seventh, too, but he wastefully side-footed Benjani's 30th-minute cut-back wide from 12 yards.
Again, Konchesky failed to test former Upton Park goalkeeper David James from long range and, in a disappointing climax to the first half, Sheringham was cautioned for dissent following one spat too many with ex-Spurs' team-mate, Sol Campbell, before Matthew Etherington followed him into the book for a frustrated lunge on Johnson.
Five minutes after the restart, Fernades drilled a low shot just wide before the dangerous Kanu sent another tightly-angled, rising volley inches over.
At the other end, Konchesky finally forced James into a save when the former England 'keeper got down at the base of his left-hand post to clutch the defender's low, 20-yarder as it flew back through the pack.
Campbell also had to be at his best to get in front of the rising Sheringham at the near post but, just as West Ham looked set to enjoy their best spell, Taylor forced Carroll to palm over his 20-yard free-kick and, from the consequent corner, the visitors did well to survive a frantic goalmouth scramble.
And after Campbell was booked for another robust aerial challenge on Sheringham, Redknapp's substitution of Kanu with the lively LuaLua, proved the cue for Pards to change his strike-force as Cole and Marlon Harewood came on for the final 20 minutes.
By now, though, referee Poll was working overtime, booking Mendes for a lunge on Reo-Coker and then the card count went to eight after Johnson and Konchesky were involved in some more unsightly pushing and shoving.
LuaLua then burst clear and fired wide with just Carroll to beat but with just 12 minutes left, Reo-Coker looked set to equalise when he let fly from 12 yards, only for the diving James to parry the shot aside at full stretch.
On 82 minutes, however, it was all over when LuaLua and Taylor combined to play in Pompey substitute Andy Cole, who climaxed his bulldozing run into the box with a low, angled 12-yard shot on the turn that beat Carroll to send Pompey up into second spot.
And this fourth successive Premiership defeat now leaves West Ham in 18th place ahead of a tricky trip to Tottenham Hotspur next Sunday.
Both Alan Pardew and former Upton Park boss, Harry Redknapp, made a trio of changes in a bid to end their sides' losing sequences.
Pards welcomed back fit-again Anton Ferdinand in place of Christian Dailly, while ex-Pompey striker Teddy Sheringham made a rare start against his old club alongside Bobby Zamora, in place of substitute Carlton Cole and the injured Carlos Tevez.
Following back to back defeats against Bolton Wanderers and Tottenham Hotspur, Redknapp recalled Manuel Fernandes, Matthew Taylor and Benjani in place of Croatian Niko Kranjcar, Sean Davis and Lomana LuaLua.
Certainly, both teams seemed intent on changing their fortunes in the early end to end - albeit niggly - exchanges, as ex-Hammer Glen Johnson fired an early shot behind, before Paul Konchesky sent a 20-yarder well wide and Pedro Mendes sent a trademark 30-yarder whistling inches wide of Roy Carroll's right-hand angle.
Both Gary O'Neil and Nigel Reo-Coker were then booked by an over-officious Graham Poll for petty fouls, before Portsmouth finally broke the deadlock midway through the half, when Kanu got between Danny Gabbidon and Ferdinand to meet Johnson's hanging right-wing cross with a stooping, far post header that eluded the tumbling Carroll.
And with the famous Pompey chimes ringing out in celebration of the Nigerian's sixth goal of the campaign, Kanu should have had a seventh, too, but he wastefully side-footed Benjani's 30th-minute cut-back wide from 12 yards.
Again, Konchesky failed to test former Upton Park goalkeeper David James from long range and, in a disappointing climax to the first half, Sheringham was cautioned for dissent following one spat too many with ex-Spurs' team-mate, Sol Campbell, before Matthew Etherington followed him into the book for a frustrated lunge on Johnson.
Five minutes after the restart, Fernades drilled a low shot just wide before the dangerous Kanu sent another tightly-angled, rising volley inches over.
At the other end, Konchesky finally forced James into a save when the former England 'keeper got down at the base of his left-hand post to clutch the defender's low, 20-yarder as it flew back through the pack.
Campbell also had to be at his best to get in front of the rising Sheringham at the near post but, just as West Ham looked set to enjoy their best spell, Taylor forced Carroll to palm over his 20-yard free-kick and, from the consequent corner, the visitors did well to survive a frantic goalmouth scramble.
And after Campbell was booked for another robust aerial challenge on Sheringham, Redknapp's substitution of Kanu with the lively LuaLua, proved the cue for Pards to change his strike-force as Cole and Marlon Harewood came on for the final 20 minutes.
By now, though, referee Poll was working overtime, booking Mendes for a lunge on Reo-Coker and then the card count went to eight after Johnson and Konchesky were involved in some more unsightly pushing and shoving.
LuaLua then burst clear and fired wide with just Carroll to beat but with just 12 minutes left, Reo-Coker looked set to equalise when he let fly from 12 yards, only for the diving James to parry the shot aside at full stretch.
On 82 minutes, however, it was all over when LuaLua and Taylor combined to play in Pompey substitute Andy Cole, who climaxed his bulldozing run into the box with a low, angled 12-yard shot on the turn that beat Carroll to send Pompey up into second spot.