The reaction mechanism for the activation of C-H bonds by coordinatively unsaturated CpM(PH3)(CH3)(+) (Cp = cyclopentadienyl; M = Rh, Ir) has been investigated by ab initio molecular orbital methods. Of the two possible mechanisms, an oxidative addition-reductive elimination process (path 1) and a sigma-bond metathesis mechanism through a four-center transition state (path 2), only the former is found for the 16-electron Ir cation, while the Rh case might adopt the latter. The reaction trajectory of path 1 for the approach of CpM(PH3)(CH3)(+) to methane and the transition state structure can be predicted on the basis of a frontier molecular orbital model that determines the orientation of attack of the CpM(PH3)(CH3)(+) fragment on a doubly occupied canonical fragment molecular orbital of methane. From which, four kinds of reaction paths (paths A, B, C, and D) can be deduced due to the asymmetric nature of CpM(PH3)(CH3)(+). Both MP2 and QCISD results suggest that path A, where the methane C-H bond breaks on the ancillary CH3 ligand side, is more favorable than other reaction paths kinetically and thermodynamically for both Rh and Ir cases. The calculational results strongly indicate that the reaction of the rhodium complex is intrinsically more difficult than that of the iridium complex. A qualitative model that is based on the theory of Press and Shaik has been used to develop an explanation for the origin of the barrier height as well as the reaction enthalpy.