Knudsen cell mass spectrometric equilibrium measurements together with theoretical computations have been employed to gain structural and thermodynamic information of the OsC and OsC2 molecules. Several levels of theory have been applied to determine the structures, molecular parameters, and physico-chemical properties of OsC(g) and OsC2(g), and their singly charged ions. Complete active space self-consistent field (CASSCF) calculations were performed on the apparent (3)Sigma (-) ground state and first (3)Delta excited state of OsC. From the analyzed gaseous equilibria and the thermal functions calculated from the computed molecular parameters, the following atomization enthalpies, Delta H-a(0)o(OsC,g) and Delta H-a(0)o(OsC2,g), and enthalpies of formation, Delta H-f(298.15)o(OsC,g) and Delta H-f(298.15)o(OsC2,g), in kJ mol(-1), have been obtained: OsC, 605.6+/-14.0 and 895.4+/-14.0; OsC2, 1154.6+/-18.0 and 1059.5+/-18.0. The results have been compared with those for the other platinum metal carbides and oxides.