An ab initio quantum chemical study of the reactions of Si+ with methylsilane has been carried out : SCF/6-31G(d) wave functions were used to predict structures of the possible products and transition states; relative energies were obtained by means of single point electron correlation corrections with fourth-order perturbation theory using the larger 6-31G(d,p) basis set. Three different mechanisms involving initial complex formation, followed by insertion of Si+ into Si-C, Si-H, and C-H bonds leading to the eliminations of H-2 and other products have been investigated in detail. This involves the detailed mapping of Si2CH6+, Si2CH5+, and Si2CH4+ potential energy surfaces. Results of the calculations are compared with the experimental observations of Mandich et al., Lim et al., and Kickel et al. Good agreement with experiments is obtained.