Osmium tetroxide reacts with mono-alkenes (R) to give five-co-ordinate mono- or di-esters of the saturated diolato-ligand (O2R)2–, viz. [Os2O4(O2R)2](I) and [OsO(O2R)2](II); (II) may also be obtained by the reaction of OsO4 with the glycol R(OH)2. With alkenes (R) or acetylene in the presence of tertiary nitrogen bases, L, the octahedral ‘osmyl’ species [OsO2L2(O2R)](III) and [Os2O4L4(O4C2H2)](IV) are formed. Spectroscopic and other data are used to assign structures to these complexes; 18O and 2H substitution has been used to help the interpretation of i.r. data.