An application of the newly developed "Integrated MO + MM (IMOMM)" method to the oxidative addition of H-2 to Pt(PR(3))(2) (R = H, Me, t-Bu, and Ph) complex is presented. In the IMOMM method, an active part of the system is handled by the MO method, the remainder is treated by the molecular mechanics force field, and the geometry is optimized fully using the sum of MO and MM energies. A comparison of full MO(RHF) results with the IMOMM(RHF:MM3) results indicates that the IMOMM method can reproduce the MO optimized geometry and energetics very well. The transition state structure for the sterically congested system changes substantially from that of the less congested system. The energetics at the more reliable IMOMM(MP2:MM3) level shows that, while for R = H, Me, and Ph the early coplanar transition state with a low barrier leads to the cis-product, for R = t-Bu the relatively late nonplanar;transition state has a high barrier, making the oxidative addition reaction difficult to take place.