The recent discovery of enzymes containing tungsten has created a new imperative for examination of key features of the structure and reactivity of biologically related compounds of this element and their relation to the corresponding properties of analogous molybdenum complexes. We have examined the relative rates of oxygen atom (oxo) transfer to substrate in the second-order reactions [MO2(mnt)(2)](2-) + (RO)(3-n)R-n'P --> [MO(mnt)(2)](2-) + (RO)(3-n)R-n'PO in DMF solution (M = Mo, W; n = 0, R = Me; n = 1, R = Me, R' = Ph; n = 1, R = Et, R' = Me). At all temperatures examined, k(Mo) > k(W); at 298 K, k(Mo)/k(W) approximate to 10(2)-10(3). For (MeO)(2)PhP as substrate, the activation parameters for M = Mo/W are Delta H double dagger = 8.2/11 kcal/mol and Delta S double dagger = -33/-38 eu. For (MeO)(3)P, the values are Delta H double dagger = 10/14 kcal/mol and Delta S double dagger = -32/-33 eu. The rate differences arise primarily because of the activation enthalpy differences Delta H-W double dagger - Delta H(Mo)double dagger approximate to 3-4 kcal/mol. Using a recent theoretical treatment of molybdenum-mediated oxo transfer to substrate (Pietsch, M. A.; Hall, M. B. Inorg. Chem. 1996, 35, 1273), an energy profile for the preceding oxo transfer reactions is proposed. Factors contributing to the difference in barriers, whose individual values fall in or close to the Delta H double dagger = 10-20 kcal/mol range for oxidation of tertiary phosphines by (MoO2)-O-VI complexes, are considered. Reducibility of the metal center and the strength of M = O bonds are two factors that will tend to differentially increase the tungsten vs molybdenum barrier. Data from other systems are assembled to support this contention. The present results suggest that, mole generally, k(Mo) > kw for atom transfer or other processes involving reduction of the metal center and the reverse for oxidation. At present, this work provides the only comparative oxo transfer rates for molybdenum and tungsten compounds with common substrates.