The effect of electronically excited oxygen O-2(a(1)Delta(g)) on the combustion of methane in air at low temperatures is investigated. Sensitivity and rate of production analysis indicated that reactions CH3 + O-2(a(1)Delta(g)) <-> CH3O2(A'); CH2O + OH; CH3O + O are important for low-temperature methane oxidation. The potential energy surface of the reactions was investigated at the multi-reference configuration interaction level of theory. It was found that the association reaction corresponding to the channel of excited CH3O2(A') formation has a threshold with an energy of 8.5 kcal/mole. The rate parameter of the reaction CH3 + O-2(a(1)Delta(g)) -> CH3O2(A') was refined and makes k = 2.76 x 10(-7) x T-2.71 x exp (- 861 kcal/mole/RT). The influence of O-2(a(1)Delta(g)) on the rate of low-temperature methane oxidation in the refined model was analyzed. The study indicates significant suppression of the contribution of the channel CH3 + O-2(a(1)Delta(g)) -> CH3O2 due to peculiar properties of the potential energy surface of the reaction with participation of O-2(a(1)Delta(g)). At the same time, high contribution of the reaction channels leading to CH2O and CH3O results in effective use of the energy of electronic excitation for acceleration of the methane oxidation in presence of the O-2(a(g)(1 Delta)) at low temperatures. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.