Gloucestershire 246 for 6 (van Buuren 61, Hammond 48) beat Somerset 242 for 9 (Gregory 52, Howell 3-45) by four wickets
Gloucestershire ended Somerset's 100 per cent Royal London One-Day Cup record with a four-wicket win in a low-scoring contest at Bristol.
The visitors posted 242 for 9 from their 50 overs after winning the toss, with Lewis Gregory their only batsman to look comfortable on the slow pitch, making 52 off 33 balls, while Benny Howell claimed 3 for 45 from his ten overs.
The result left the hosts with six points from five matches and Somerset eight from the same number of games.
The first ten overs of Somerset's innings brought just 32 runs for the loss of Tom Banton, bowled by a David Payne yorker for 11. It was 38 for 2 when Peter Trego fell lbw attempting to sweep left-arm spinner van Buuren.
James Hildreth chipped a catch to mid-wicket off Ryan Higgins with the total on 51 and Azhar Ali had failed to hit a single boundary when bowled trying to cut loose with a reverse sweep off Tom Smith for 43.
Somerset managed only two fours off the bat in the first 30 overs, which ended with them 105 for 4. Skipper Tom Abell and George Bartlett added 55 before the latter was run out for 29 by Higgins attempting a second run to deep point.
Gregory announced his intentions with a six over long-on off Payne, but Somerset still needed an injection of quick runs when Abell was caught at backward-point off Smith for 42. Gregory hit sixes off successive balls from Higgins, before Roelof van der Merwe was caught at third-man off Howell for 16.
Gregory's impressive contribution was ended by a catch at mid-off, giving Howell his second wicket. Craig Overton was unbeaten on 25.
Gloucestershire's reply was off to a bad start when, in the first over, skipper Chris Dent was run out at the non-striker's end by bowler Overton, sent back attempting a quick single.
Soon Gloucestershire were finding it equally difficult to score quickly as they crawled to 26 for 1 from ten overs. When Gregory struck with successive deliveries in the 14th over, having Gareth Roderick caught behind for 25 and Howell caught at point for a first-ball duck, the outcome was in the balance.
After 25 overs, Gloucestershire were 86 for 3. Then James Bracey, on 21, attempted to launch Josh Davey over the leg-side and fell to a well-judged catch by van der Merwe.
Miles Hammond played sensibly for his 48 off 83 balls when brilliantly caught by Azhar at mid-wicket off Tim Groenewald and Gloucestershire were in trouble at 114 for 5 in the 31st over.
Somerset's bowlers were backed up by some inspired fielding. With 15 overs to go, Gloucestershire needed a further 115 at seven and half an over. Their hopes rested largely with van Buuren and Taylor, who both hit sixes off Davey to boost the run-rate. Neither looked troubled as van Buuren moved to a fine half-century off 52 balls before miscuing Overton to Azhar at mid-off.
The pair had added 83 in 14 overs. Higgins blasted Groenewald over long-on for six as the 46th over went for 17. Another massive six by Higgins off the same bowler two overs later sealed Somerset's fate, Groenewald losing his line and length to prove expensive.