Oxidative addition of the Si-X sigma-bond (X = H, C, Si) to Pd(PH3)2 is theoretically investigated with the ab initio MO/MP4 method. The activation barrier (E(a)) and the energy of reaction (DELTAE) estimated at the MP4(SDQ) level are 13.5 and 18.1 (19.6 and -46.4) kcal/mol, respectively, for the Si-Si oxidative addition, 21.5 and 8.1 (29.5 and -14.1) kcal/mol, respectively, for the Si-C oxidative addition, and about 2 and -8.1 (1.8 and -31.1) kcal/mol for the Si-H oxidative addition, where in parentheses are given E(a) and DELTAE of the reaction with Pt(PH3)2 (from our previous work published in J. Am. Chem. Soc. 1993, 115, 2373) and a negative DELTAE value represents exothermicity. Although Si-X oxidative addition to Pd(PH3)2 is much less exothermic (or much more endothermic) than the corresponding oxidative addition to Pt(PH3)2, the E(a) value of the former is slightly smaller than (or similar to) that of the latter, suggesting that the Si-X oxidative addition to Pd(PH3)2 as well as its oxidative addition to Pt(PH3)2 easily proceeds in spite of the smaller exothermicity. The smaller exothermicity (or larger endothermicity) of the Si-X oxidative addition to Pd(PH3)2 is because the Pd-X bond is weaker than the Pt-X bond. The reason for the rather small E(a) value is that Pd(PH3)2 causes distortion (the PPdP bending) with a smaller destabilization energy than Pt(PH3)2 and that the d(HOMO) orbital of Pd(PH3)2, taking a distorted structure as in the TS, lies at relatively high energy, simialr to the d(HOMO) orbital of Pt(PH3)2.