In this study, the reaction mechanism of toluene 1,2-epoxide/2-methyloxepin with OH radical was studied by means of quantum chemical computations performed using B3LYP/6-31G(d,p), B3LYP/6-311G(2df,2p), and BHandHLYP/6-31G(d,p) methods. Ground state, intermediate, and transition states were determined. The results indicated that the 2-methyloxepin, A, isomer is more stable, by 2.4 kcal/mol, than toluene 1,2-epoxide, B. Two reaction pathways were studied, RP-A and RP-B, corresponding to the reaction of OH with toluene 1,2-epoxide and 2-methyloxepin, respectively. The localization of a pre-reactive complex for RP-A is crucial for the accurate estimation of the rate constant, k = 1.0 x 10(-10) cm(3) molecule(-1) s(-1), which is in good agreement with that determined experimentally, whereas for RP-B the rate constant is 1.3 x 10(-14) cm(3) molecule(-1) s(-1). Under atmospheric conditions, both pathways yield 6-oxohepta-2,4-dienal as a main product, and from the energetic and kinetic results it was found that R-P-A is the preferred pathway. The study of the oxide/oxepin mechanism is relevant because, aside from its relatively high concentration in the troposphere, this compound has carcinogenic and mutagenic properties.