CLYDESDALE BANK 40 GROUP A, The Oval:
Worcestershire 376-6 beat Surrey 286 by 90 runs
Pietersen drives through the off side during his 38 at The Oval
Kevin Pietersen missed out on a big score on his debut for Surrey, who now cannot reach the CB 40 semi-finals after a 90-run loss to Worcestershire.
The England batsman smashed a return catch to left-arm spinner Shaaiq Choudhry to fall for 38 from 33 balls.
Earlier, Vikram Solanki hit 129 and Gareth Andrew 104, his first one-day century, in the visitors' 376-6, their new county record 40-over score.
Captain Rory Hamilton-Brown then hit 80 as Surrey were all out for 286.
Pietersen has joined Surrey on a month's loan from Hampshire after being left out of the England one-day squad.
But, in his first game of domestic 40-over cricket since 2005, he was denied an early chance to find some pre-Ashes form when Worcestershire captain Daryl Mitchell won the toss and elected to bat on a flat Oval pitch.
The visiting batsmen made the most of the opportunity, first in the shape of Mooen Ali, who hit 41 from 28 balls in an opening stand of 61 with Solanki.
Jack Manuel and Alexei Kervezee failed to take advantage of the perfect batting conditions but Solanki and Andrew cashed in, sharing 185 for the fourth wicket.
Former captain Solanki hit 13 fours and three sixes in his 89-ball knock, while Andrew, batting with the long-handled Mongoose bat, cleared the ropes six times.
Two of those maximums came from Pietersen's sole over, which cost 17 runs and also saw Andrew put down by a diving Matthew Spriegel at long-off.
Andrew's 104 off 60 balls also included eight fours, as well as his six sixes, as he smashed his one-day best for the second time in 10 days.
He had only passed 50 for the first time in a one-day game when he made 66 not out off just 26 balls against Glamorgan at New Road on 22 August.
Worcestershire's massive total was their highest ever one-day score against a first-class county, also surpassing their best 40-over total - 316-5 against Gloucestershire, also at New Road, in 2008.
Surrey were given a blistering start by Hamilton-Brown but his opening partner Steven Davies, who had already plundered 386 runs in six innings against his old county this season, failed for the first time.
Davies was cleaned up by Jack Shantry to bring Pietersen to the crease, Surrey's new loan star immediately getting off the mark with an unconvincing single through the leg side. But he was quickly into his stride with a cover drive for four off Shantry's next over.
Pietersen twice swept Choudhry for four, but was starved of the strike by skipper Hamilton-Brown in a second-wicket stand of 79.
Surrey kept up with the required rate as Hamilton-Brown took the batting powerplay but holed out to long-on off Choudhry.
Pietersen enjoyed a few slices of luck as mistimed drives landed between fielders but his good fortune was ended when he made room outside the leg stump to drill the ball back to Choudhry.
After that, some late-order hitting from Chris Schofield, Surrey's other Worcestershire old boy Gareth Batty and Chris Tremlett gave Surrey some cheer, but they eventually fell to defeat.