An ab initio molecular orbital method at the MP2 level of theory in conjunction with a relativistic core potential and valence triple-zeta + polarization basis set for Rh and double-zeta + polarization basis set for other atoms has been applied to the study of the potential energy surface of the oxidative addition reaction CpRh(CO) + HR --> CpRh(CO)(H)(R), where HR is H-2, CH4, NH3, H2O, and SiH4. At gas-phase collisionless conditions, the oxidative addition reaction of H-SiH3, H-H and H-CH3 to CpRh(CO) should take place without an activation barrier, while the reaction of H-NH2 and H-OH goes over a barrier about 5 kcal/mol relative to the reactants. The differences in the reactivity of the substrates considered here can be correlated to the H-R bond strength and the Rh-R bond strength as well as the exothermicity of reaction. Going from SiH4 to H-2, CH4, NH3, and H2O, the H-R bond becomes stronger (88, 99, 108, 109, and 118 kcal/mol, respectively, calculated at the present level), the Rh-R bond becomes weaker (73, 65, 59, 47, and 55 kcal/mol, respectively), the exothermicity becomes smaller (49, 31, 16, 3, and 2 kcal/mol, respectively), and the ease of reaction decreases. In solution or in the gas phase when the collisional energy equilibrium is faster than the reaction itself and reaction should be considered to start from the pre-reaction molecular complex CpRh(CO) (HR), the oxidative addition reaction of CH4 requires a small barrier (6 kcal/mol), while that of NH3 and H2O requires a large barrier (42 and 26 kcal/mol, respectively) and would not take place easily under normal conditions. The high barrier is essentially determined by the stability of the molecular complex.