The complex [HOs3(CO)10(SPh)] readily dissolves in concentrated sulphuric acid to produce the cationic hydrido-species [H2Os3(CO)10(SPh)]+(α-form). In the n.m.r. spectrum two resonances are observed in the high-field region and on this basis the hydrido-ligands are considered to be nonequivalent. On heating the solution at 100 °C for 1 h a new isomer is produced (β-from). The α-form does not undergo proton exchange with sulphuric acid but there is exchange of hydrogen between the two nonequivalent sites. Reaction of HOs3(CO)10(SPh) with triethylphosphine produces three products which could be isolated,viz. [HOs3(CO)9(PEt3)(SPh)] and [HOs3(CO)8(PEt3)2(SPh)](two isomers), together with products of further substitution. With methyldiphenylphosphine similar products were observed but only [HOs3(CO)9(PMePh2)(SPh)] in two isomeric forms was isolated. Protonation of these derivatives occurs in sulphuric acid to give e.g.[H2Os3(CO)9L(SPh)]+(L = PEt3 or PMePh2) and [H2Os3(CO)8(PEt3)2(SPh)]+ which could be isolated as hexafluorophosphate salts; their possible structures are discussed. Evidence for the doubly protonated species [H3Os3(CO)8(PEt3)2(SPh)]2+ is also given.