The water-soluble tungsten complex: [W(CO)(3)(MeCN)(TPPMS)(2)] where TPPMS = (C6H5)(2)P(m-C6H4SO3Na . 2H(2)O), was prepared and characterized using spectroscopic techniques. The complex is soluble and stable in water for long periods as demonstrated by UV-VIS studies. When this water-soluble tungsten complex was used as a catalyst precursor during the two-phase catalytic hydrogenation of small olefins and aromatic molecules (I.-hexene, styrene, 2,3-dimethyl-1-butene, 2,3-dimehtyl-2-butene and benzene) the expected selectivity for a homogeneous catalyst precursor was not obtained. Under the chosen reaction conditions, the hydrogenation of benzene proceeded at a much faster rate (benzene: 2.1; 1-hexene: 1.2; styrene: 1.3; 2,3-dimethyl-1-butene: 1.5 and 2,3-dimethyl-2-butene: 1.6 mol(mol W)(-1) h(-1)). The Hg test, carried out in another set of experiments, showed inability of the system for catalytic hydrogenation. Upon changes in the pH of the catalytic solution, the hydrogenation proceeded either homogeneously or heterogeneously. At pH < 6, the catalysis proceeds heterogeneously, while at pH > 6 the process is homogeneous (Scheme 1). Hydrogen pressure also facilitates this transition. Temperature, substrate to catalyst ratio (S/C) and stirring do not affect the heterogeneous nature of the system. The heterogeneous catalytic solution is blue while the homogeneous one remained yellow.