Semiempirical configuration interaction (SECI) theory to predict activation barriers, E, as given by k(T) = AT(n) exp(-E(RT), has been applied to homologous series of lanthanide (LN) and transition metal (TM) atom oxidation reactions. This was achieved by considering as homologous series reactions of elements differing only by the number of electrons in one subshell. Comparison between SECI and experimental results leads to an average deviation for the LN+N2O reactions of 0.66 kJ mol(-1), and up to 5.5 kJ mol(-1) for other series. Thirty-one activation barriers are reported.