We performed the first theoretical potential energy surface investigation on the mechanism and products of the reaction of OH+ cyclopentene in the absence and presence of O-2 by using high-level quantum chemical methods CCSD(T)/6-311++G(d,p)//BH&HLYP/6-311++G(d,p)+ZPE x 0.9335. Energies for several species are also refined at the CCSD(T)/cc-pVTZ levels of theory. The calculations indicate that the major products are cyclopentanone, 1-cyclopenten-1-ol, and 2-cyclopenten-1-ol in the absence of O-2, which are in qualitative accordance with the available experimental observations. In the presence of O-2, the dominant products are predicted to be glutaraldehyde and 1,2-epoxycyclopentanol. (C) 2013 Elsevier B.V. All rights reserved.