The equilibrium geometries of the molybdenum oxo/peroxo compounds MoOn(O-2)(3-n) and the related complexes [MoOn(O-2)(3-n)(OPH3)] and [MoOn(O-2)(3-n)(OPH3)(H2O)] (n = 0-3) have been calculated using gradient-corrected density-functional theory at the B3LYP level. The structures of the peroxo complexes with ethylene ligands [MoOn(O-2)(3-n)(C2H4)] and [MoOn(O-2)(3-n)(OPH3)(C2H4)] (n = 1, 2) where ethylene is directly bonded to the metal have also been optimized. Calculations of the metal-ligand bond-dissociation energies show that the OPH3 ligand in [MoOn(O-2)(3-n)(OPH3)] is much more strongly bound than the ethylene ligand in [MoOn(O-2)(3-n)(C2H4)]. This makes the substitution of phosphane oxide by olefins in the epoxidation reaction unlikely. An energy-minimum structure is found for [MoO(O-2)(2)(OPH3)(C2H4)], for which the dissociation of C2H4 is exothermic with D-0 = -5.2 kcal/mol. The reaction energies for the perhydrolysis of the oxo complexes with H2O2 and the epoxidation of ethylene by the peroxo complexes have also been calculated. The peculiar stability of the diperoxo complex [MoO(O-2)(2)(OPH3)(H2O)] can be explained with the reaction energies for the perhydrolysis of [MoOn(O-2)(3-n)(OPH3)(H2O)]. The first perhydrolysis step yielding the monoperoxo complex is less exothermic than the second perhydrolysis reaction, but the further reaction with H2O2 Yielding the unknown triperoxo complex is clearly endothermic. CDA analysis of the metal-ethylene bond shows that the binding interactions an mainly caused by charge donation from the ligand to the metal.