The reactive vaporization of MnO(c) in the presence of O-2 and H2O/D2O has been studied by effusion-beam mass spectrometry, and the new species MnO2, MnOH, Mn(OH)(2), and MnO(OH) have been identified. The identities of the hydroxides were confirmed:from deuterium isotopic substitution. Reaction enthalpies and standard enthalpies of formation have been. derived from equilibrium measurements, yielding the Delta(f)H degrees(298) values for;the gaseous species MnO2, -17.6; MnOH, 0.1; Mn(OH)(2), -95.9; and MnO(OH), -41.1, all in kcal mol(-1). For use in-the analysis, thermal functions were calculated from known or estimated molecular constants. Bond energies derived from the results fall in a logical and consistent sequence.