West Brom striker Peter Odemwingie made it a miserable return to the Hawthorns for Chelsea manager Roberto Di Matteo with a second-half winner to stun the Champions League holders 2-1.
The Nigerian international headed home a cross from Shane Long whose opening goal had been cancelled out by Eden Hazard.
It condemned Di Matteo, who spent two years as Albion boss, and his side to a first away Barclays Premier League defeat of the campaign.
The Baggies took the lead via their first worthwhile attack after nine minutes. Zoltan Gera played the ball back to James Morrison whose floated cross picked out Long who got in behind David Luiz to head home from close range.
Chelsea tried to retaliate and John Obi Mikel's low 30-yard drive was only just off target before Boaz Myhill then got his body behind a swerving first time attempt from Daniel Sturridge.
Albion were at their most threatening on the counter-attack and Long, under pressure from Gary Cahill, had a shot saved by Petr Cech.
Long was then just off target with a curling drive after he had shaken off the challenge of Cahill but after 38 minutes Hazard levelled for the Blues with his fourth goal of the season. He attacked Cesar Azpilicueta's deep cross and was able to squeeze the ball past Myhill with a powerful header.
Chelsea continued to dominate possession and Victor Moses fired over but Odemwingie restored Albion's lead five minutes into the second half.
This time Long was the creator with a cross to the near post and the Nigeria international got in front of Ryan Bertrand to glance a header past Cech.
Myhill made a fine save to turn aside Sturridge's flick and then clung on to Hazard's shot.