Diphenylacetylene and 1-phenyl-l-propyne were hydrogenated to the corresponding 1,2-disubstituted alkenes in aqueous organic biphasic media using the water-soluble catalyst [(RuCl2(mtppms)(2))(2)] and an excess of the sulfonated phosphine ligand. The stereoselectivity of the reaction strongly depends on the pH of the catalyst-containing aqueous phase and under acidic conditions Z-alkenes can be obtained with a selectivity close to 100%.