Three defensive errors handed West Ham victory despite a late Arsenal comeback in a lively encounter at Highbury.
Reo Coker celebrates West Ham's opener
Nigel Reo-Coker capitalised on Sol Campbell's mis-kick on 25 minutes before Bobby Zamora shrugged off Campbell and curled home for 2-0.
Thierry Henry became the Gunners' leading league scorer by deflecting in a Robert Pires drive before the break.
But when Matthew Etherington stretched the Hammers' lead late on, Pires' goal became only a consolation.
Arsenal made four changes to the side that lost to Bolton in the FA Cup on Saturday with midfielder Abou Diaby making his full Premiership debut.
It was the home side who dominated the opening quarter as they pinned the visitors in their own half in a high-tempo start.
Robin van Persie's trickery almost broke the deadlock in the seventh minute, but his close range effort crashed against the upright after weaving through the Hammers' defence.
But it was the away side who took the lead on 25 minutes when Campbell's horrific attempted clearance allowed Reo-Coker a free run on goal from which the skipper coolly slotted past Jens Lehmann.
The Hammers then doubled their lead when Zamora shrugged off Campbell in the box and curled his effort superbly into the Arsenal net on 32 minutes.
Despite record-signing Ashton taking his place on the West Ham bench, the powerful front two pairing of Bobby Zamora and Marlon Harewood continued to cause problems to the Arsenal back-line.
But Arsenal responded just before the break when West Ham failed to clear their lines from a Freddie Ljungberg run.
Pires' low drive through a crowd of players deflected in off Henry to give the Frenchman the league goals record for the club, overtaking Cliff Bastin's total of 150.
Campbell failed to re-appear for the second half, handing youngster Sebastian Larsson a rare start and a further reshuffle to the Arsenal defence.
Arsenal started the second half with the same verve as they began the first with Pires and Ljungberg wasting good chances and van Persie causing problems from set pieces.
Henry then broke free inside the area but saw his bending effort whistle past the far post.
But Arsenal failed to find the net and Matthew Etherington stretched the Hammers' lead after Larsson and Phillipe Senderos were caught in possesion in defence.
Ashton came on for his debut in place of goal scorer Zamora.
Pires pulled one back on 90 minutes to set up a tense finish, but the Hammers held on for a hard-fought victory.
Arsenal manager Arsene Wenger:
"We lost and it's difficult to understand how we lost because their keeper
was very busy and our keeper was not busy.
"Overall, commitment was right, the quality of our game was right and we
succumbed to basically mistakes that can of course happen in football."
West Ham manager Alan Pardew:
"We set our stall out we rode our luck at times. We mixed it up a bit and got goals. It was a fantastic performance.
"Their back four hasn't played together and we knew we had to expose that, we were always going to threaten them.
"But they really terrorised us for the first 10 minutes and we needed to be strong and our centre halves were immense tonight."
Arsenal: Lehmann, Gilbert (Flamini 27), Djourou, Campbell (Larsson 45), Senderos, Ljungberg, Diaby (Bergkamp 71), Silva, Pires, Henry, Van Persie.
Subs Not Used: Almunia, Hleb.
Goals: Henry 45, Pires 89.
West Ham: Hislop, Clarke (Fletcher 76), Ferdinand, Gabbidon, Konchesky, Benayoun (Newton 66), Mullins, Reo-Coker, Etherington, Harewood, Zamora (Ashton 73).
Subs Not Used: Bywater, Sheringham.
Goals: Reo-Coker 25, Zamora 32, Etherington 80.
Ref: M Halsey (Lancashire).