A theoretical study of oxidative additions of H-CH3, CH3-CH3, H-SiR3, and SiR3-CH3 (R=H, Cl, or Me) to Pt(PH3)(2) was carried out with ab initio MO/MP2-MP4SDQ, CCD, and CCSD methods. The oxidative addition reactions of C-H and Si-H a-bonds occur through a planar transition state (TS) structure, in accordance with the expectation from an orbital interaction diagram. However, the oxidative addition reactions of CH3-CH3 and SiH3-CH3 take place through a nonplanar TS structure, unexpectedly; the dihedral angle delta between PtP2 and PtXC planes (X = C or Si) is about 70 degrees for X = Si and about 80 degrees for X = C. Intrinsic reaction coordinate calculation of the SiH3 -CH3 oxidative addition clearly indicated that this nonplanar TS is smoothly connected to the planar product on the singlet surface. The dihedral angle delta at the TS is larger in the SiMe3-CH3 and SiCl3-CH(3)oxidative additions than that in the SiH3-CH3 oxidative addition. Electron distribution in the TS and effects of bulky substituent on the dihedral angle suggest that not an electronic factor but a steric factor is responsible for the nonplanar TS structure of the C-C and Si-C oxidative addition reactions.