The increase of the rate constants with the degree of methylation and the negative temperature dependence in the·OH + alkenes reactions has been investigated using the CCSD(T)/6‐311++G(2d,2p)//BHandHLYP/aug‐cc‐pVTZ potential energy surface and the canonical transition state theory. Our results show that the pre‐exponential factor has positive temperature dependence for all the studied reactions. The experimental finding that the reaction rates augment as temperature decreases and the methylation degree increases is explained by the effective negative enthalpy of activation due to a complex mechanism, as first proposed by Singleton and Cvetanovic. The excellent agreement between the calculated and experimental rate constants at 298.15 K strongly supports our conclusions. © 2012 Wiley Periodicals, Inc.