A theoretical study on the mechanism of the OH + aliphatic thiols reactions is presented. Optimum geometries and frequencies have been computed at the 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(d,p). Twelve possible channels have been modeled, three of them including the possible influence of molecular oxygen, and three of them involving excess of OH. The only channels that have been found to significantly contribute to the overall reaction in the troposphere are the hydrogen abstractions from the -SH group and from the alkyl groups. Our analysis supports a stepwise mechanism involving the formation of a short-lived, weakly bonded adduct in the entrance channel, for the abstraction paths. The results proposed in the present work seem to provide a viable explanation for diverse findings previously reported from experimental investigations.