A theoretical study on the mechanism of the OH reactions with HCN and CH3CN, in the presence of O-2, is presented. Optimum geometries and frequencies have been computed at BHandHLYP/6-311++G(2d,2p) level of theory for all stationary points. Energy values have been improved by single-point calculations at the above geometries using CCSD(T)/6-311++G(2d,2p). The initial attack of OH to HCN was found to lead only to the formation of the HC(OH)N adduct, while for CH3CN similar proportions of CH2CN and CH3C(OH)N are expected. A four-step mechanism has been proposed to explain the OH regeneration, experimentally observed for OH + CH3CN reaction, when carried out in the presence of O-2. The mechanism steps are as follows: (1) OH addition to the C atom in the CN group, (2) O-2 addition to the N atom, (3) an intramolecular H migration from OH to OO, and (4) OH elimination. This mechanism is in line with the one independently proposed by Wine et al.(11) for HCN. The results obtained here suggest that for the OH + HCN reaction, the OH regeneration might occur even in larger extension than for OH + CH3CN reaction. The agreement between the calculated data and the available experimental evidence on the studied reactions seems to validate the mechanism proposed here.