A homogeneous dihydroxylation catalyst was synthesised by complexation of osmium tetroxide (OsO4) with a silsesquioxane ligand containing a tetrasubstituted olefin moiety. The advantage of this procedure consists in avoiding the presence of the highly toxic and volatile OsO4 in solution during the dihydroxylation reaction. The Os-silsesquioxane complex was used as a homogeneous catalyst and as a model of the catalytic site of the heterogeneous analogue. The homogeneous catalyst gave 84% conversion with 99% selectivity for the dihydroxylation of cyclopentene and 99% conversion with 99% selectivity for the dihydroxylation of cyclohexene. (C) 2004 Elsevier B.V. All rights reserved.