The osmium-catalyzed dihydroxylation of various olefins using molecular oxygen or air as the stoichiometric oxidant is reported. Aromatic olefins yield the corresponding diols in good to excellent chemoselectivities under optimized pH conditions (pH = 10.4-12.0), Air can be used under moderate pressures (3-9 bar) instead of dioxygen as the reoxidant. By increasing the oxygen content of the solution, it is possible to achieve highly efficient conversion at low catalyst amount (catalyst/substrate = 1:4000). Tri- and tetrasubstituted olefins are oxidized at pH > 11 to give the corresponding 1,2-diols in good to very good yields without requiring the addition of sulfonamides or other hydrolysis agents. Studies of the dihydroxylation of functionalized olefins demonstrate that the reaction conditions tolerate a variety of functional groups. In the presence of dihydroquinine or dihydroquinidine derivatives (Sharpless ligands), asymmetric dihydroxylations occur with lower enantioselectivities than tose of the classical K-3[Fe(CN)(6)] reoxidation system.