The dehydrogenation of cyclopentene (c-C5H8) to cyclopentadienyl (c-C5H5) on Pt(111) is studied using a semiempirical molecular orbital method, Two possible mechanisms of reaction are considered, Results show that a sequential mechanism is favored over a simultaneous one, We also find a strong parallelism with previous theoretical results for c-C6H12 dehydrogenation to benzene, A bonding mechanism dominated by electron donation from pi orbitals of c-C5H5 into metal orbitals is found, Similar results have been reported for benzene adsorbed on Pt(111). (C) 1999 Elsevier Science B.V. All rights reserved.