Reaction of BiX3 (X = Cl, Br, I) with [PtMe2(P-P)], (1a, P-P = dppm; 1b, P-P = dppe), occurs easily to yield a mixture of platinum(II) complexes [PtMeX(P-P)], 2, and [PtX2(P-P)], 3, and the binuclear complex [Pt2Me2(mu-X)(mu-dppm)(2)]X, 4. On the basis of P-31 NMR and UV-vis spectroscopy, a mechanism is proposed in which the rate determining step is conversion of the yellowish Pt(II)-BiX3 adduct BiI3 center dot[PtMe2(dppm)], A, into the Pt(IV)-Bi(III) intermediate [PtMe2(BiX2)X(P-P)], IM1. Density functional theory (DFT) studies suggest that intermediate IM1 may be formed in acetone solution which undergoes the Bi-C reductive elimination process before formation of complexes 2 and 3. The structures of intermediates IM1 were theoretically determined using DFT calculations. In dilute acetone solution, as monitored by UV-vis spectroscopy, the oxidative addition processes follow first order kinetics. The overall reaction is slower for heavier halide.