Bucks put up a bold display but lost by 115 runs to a strong Sussex side in their showpiece game at Hove on Tuesday.
With former England seamer Steven Finn in their near first-choice line-up, Sussex chose to bat first and rattled up 335-9 from their 50 overs on an easy-paced pitch.
Top scorer was 19-year-old James Coles, son of the former Bucks player and team manager Jon Coles, who made a classy 61 from 42 balls. He was supported by half-centuries from skipper Tom Haines (60), Tom Alsop (54) and Tom Clark (53).
Both skipper Tom Hampton and seamer Conner Haddow took three wickets, while their High Wycombe teammate Cameron Parsons was Bucks's most economical bowler. In addition, AJ Woodland and Ed Bragg took excellent catches in the deep in a fine team fielding display.
Even though Jofra Archer was limited to warming up on the sidelines, Bucks never looked like reaching their target, after Woodland fell to Finn and Alexei Kervezee departed cheaply.
Buckingham opener Ewan Cox struck two sixes in his 44 from 56 balls, while Great Brickhill's 17-year-old Aadi Sharma made a stylish 46 from 62 balls.
A few lusty blows from Haddow who hit two sixes and a reverse sweep for four in his unbeaten 31 off 22 balls, helped Bucks to reach 220 all out with 6.2 overs to spare.
Scoreboard
Bucks now turn their attention to their National Counties Championship match against Lincolnshire at Chesham, starting on Sunday.
Having won their opening two matches, Bucks stand well clear of the Eastern Division 1 table with two games to play.
Another victory would give them a great chance of challenging for the overall Championship title.
Bucks squad: T Hampton (High Wycombe) capt, A Woodland (High Wycombe), E Cox (Buckingham), C Marrow (Gerrards Cross), A Kervezee (Halesowen), R Richardson (Blackheath), E Bragg (Halesowen), H Ramanadanpulle (Buckingham), C Parsons (High Wycombe), C Haddow (High Wycombe), T Weymes (Tring Park)