The potential energy surface for the reaction of cyclopentadienyl radical with O-2 has been studied using ab initio calculations at the CCSD(T)F12/cc-pVTZ-f12//B3LYP/6-311G(d,p) level and the RRKM-Master Equation approach has been employed to compute reaction rate constants and product branching ratios at various temperatures and pressures pertinent to combustion. The results show that at low temperatures from 500 to 800-1250 K (depending on pressure), the reaction predominantly forms a collisionally-stabilized C5H5-OO complex and then, the thermalized complex rapidly decomposes back to the reactants establishing a C5H5+O-2/C5H5-OO equilibrium. At higher temperatures, typically above 1000K, the mechanism is different and the C5H5 + O-2 reaction proceeds to form various bimolecular products. Cyclopentadienone C5H4O + OH are predicted to be the predominant product (63.5-83.3%). Relatively minor products include H2CCHCHC(H)O + CO (20-3%), vinylketene + HCO (12-2%), and OC(H)CHCHCHCO+H (35%), which are formed via the OC(H)CHCHCHC(H)0 intermediate residing in a deep potential well, and highly endothermic C5H5O + O (up to 6.5% at 2500K) produced directly by the O-O bond cleavage in the initial complex. The calculated rate constants for the formation of C5H4O +OH and C5H5O + O are shown to be independent of pressure above 800 K, but the rate constants for the reaction channels resulting in CO, HCO, and H eliminations show some pressure dependence in the low end of the high-temperature regime and decrease with the pressure growing from 10 to 100 atm. The CO2 loss channel leading to the formation of 1,3-butadien-1-yl C4H5 is shown to be negligible. The total reaction rate constants at all considered pressures from 0.03 to 100 atm merge at 1375 K and show no pressure dependence at higher temperatures, as only the bimolecular products are formed. Overall, the rate constant of the C5H5 +O-2 reaction at combustion-relevant temperatures is predicted to be very slow, 10(-16)-10(-15) cm(3) molecule(-1) s(-1), that is typically similar to 5 orders of magnitude lower than those for the oxidation reactions of cyclopentadienyl with OH and O(P-3). A comparison of the rates of the C5H5+ O-2/OH/O reactions allowed us to conclude that molecular oxygen can play only a small role in oxidation and removal of five-member rings in combustion and only when the concentration of O-2 is orders of magnitude higher than the concentrations of O and OH. (C) 2018 The Combustion Institute. Published by Elsevier Inc. All rights reserved.