Reaction of the zerovalent osmium compound, Os(CO)3(PPh3)2, with a number of strong acids has produced cationic hydrido-complexes of the type [OsH(CO)3(PPh3)2]X (X–= HCl2–, Br–, ClO4–, BF4–, or PF6–). I.r. spectra in the carbonyl-stretching region indicate a trans arrangement of the phosphine ligands. The compounds behave as 1 : 1 electrolytes in nitrobenzene and can be reconverted to Os(CO)3(PPh3)2 by the action of bases. Carbonyl groups are not displaced from the cation by an excess of methyldiphenylphosphine under vigorous conditions, and the hydrido-cation is inert even to phosphine exchange. The neutral hydride, OsHCl(CO)2(PPh3)2 is also reported together with several new synthetic routes to the dihydride, OsH2(CO)2(PPh3)2, including one by decarboxylation of the diformate, Os(HCO2)2(CO)2(PPh3)2.