A theoretical study of the reactions of CH3+ with second-row atoms in their ground states, which are interesting in general gas-phase chemistry but particularly in interstellar chemistry, has been carried out. For this purpose ab initio molecular orbital calculations of the (XH(n)C)(+) systems, X being Si, P, S, and Cl (n = 1-3), have been made. The lowest-lying stable species of each system have been characterized, and we have searched for all the relevant saddle points. We have found three exothermic channels for the reaction of CH3+ With Si : the electron-transfer process, production of SiCH2+(B-2(1)) + H(S-2), and production of SiCH+((3) Sigma(+)) + H-2((1) Sigma(g)(+)). We have found that quenching into Si+(P-2) + CH3((2)A(1)) should be a likely process when the system follows the lowest (3)A" potential energy surface and that there are two barrier-free paths for the channel giving SiCH2+(B-2(1)) + H(S-2). Production of SiCH+((3) Sigma(+)) + H-2(1 Sigma(g)(+)) is only slightly exothermic, and one of the transition states involved lies below the reactants by a very small quantity, so it is hard to make a definitive statement on whether the process might take place. The reaction of CH3+ With P has no exothermic reaction channels within the quartet state. However, there are indications that spin flipping into the doublet state is a likely process, opening a possibility for the production of PCH2+((1)A(1)) + H(S-2). For the reactions of CH3+ with S and Cl there are two exothermic channels in each case, but only those giving SCH2+(B-2(2)) + H(S-2) or ClCH2+((1)A(1)) + H(S-2), respectively, are not subject to energy barriers. The electron-transfer reaction is endothermic in both cases, whereas production of SCH+((3)A(1)) + H-2((1) Sigma(g)(+)) or ClCH+((2)A’) + H-2((1) Sigma(g)(+)) is prevented by small barriers.