A theoretical study of model OsCl2(PH3)(2) complex and its adducts with hydrogen and methane is reported. Geometry of two isomers of OsCl2(PH3)(2) complex and structures of possible sigma complexes (eta(2)-H-2)OsCl2(PH3)(2), (eta(2)-CH4)OsCl2(PH3)(2) have been calculated using density functional theory and MP2-method. It was shown that some isomers of weakly bonded hydrogen sigma-complex OsH2Cl2(PH3)(2) can form. stable methane adducts followed by reductive elimination of hydrogen. Calculations of the reaction pathways for methane oxidative addition to isomers of 14-electron intermediate OsCl2(PH3)(2) have been performed. For the isomer active in this reaction, structures of the transition state and methylhydride product were calculated. Frequencies of vibrations of stable sigma-complex (eta(2)-CH4)OsCl2(PH3)2 along with its deuterated form (eta(2)-CD4)OsCl2(PH3)(2) have been computed in the MP2/LAN1DZ approximation.