Osmium has been inserted into macrocyclic tertiary amine ligands [L = 1,4,8,11-tetramethyl-1,4,8,11 -tetra-azacyclotetradecane, 1,4,8,12-tetramethyl-1,4,8,12-tetra-azacyclopentadecane, 1,5,9,13-tetramethyl-1,5,9,13-tetra-azacyclohexadecane, and meso-2,3,7,11,12-pentamethyl-3,7,11,17-tetra-azabicyclo[11.3.1 ]heptadeca-1 (17),13,15-triene] to give trans-[OsIIILCl2]+, which could be oxidized to trans-[OsVIL(O)2]2+ by H2O2. The trans-[OsVIL(O)2]2+ complexes show one intense νasym(OsO2) stretch at 870–880 cm–1. Their u.v.–visible spectra display two vibronic structured electronic absorption bands centred at ca. 310 and 350 nm, assignable to the spin-allowed and spin-forbidden (dxy)2→(dxy)1(dπ*)1transitions (dπ*=dxz′dyz). Room-temperature photoluminescence with emission maxima in the region 620–710 nm from trans-[OsVIL(O)2]2+ has been observed. In acetonitrile, trans-[OsVIL(O)2]2+ displays reversible OsVI–OsV and Os V–OsIV couples with E⊖ ranging from –0.67 to –0.73 and –1.48 to –1.74 V vs. ferrocenium–ferrocene, respectively. The complexes trans-[ OsVL(O)2]+ have been characterized by u.v.–visible spectroscopy and magnetic susceptibility measurements. In aqueous solutions, pH-dependent and reversible OsVI–OsIII couples have been observed. Thermally, trans-[OsVIL(O)2]2+ complexes are poor oxidants for alkene oxidation.