We synthesized a new water soluble ruthenium complex which contains the hydrophilic ligand tris(3-hydroxypropyl)phosphane. This compound catalyzes the regioselective biphasic hydrogenation of sorbic acid to yield 4-hexenoic acid with 82% selectivity and with turnover frequencies of up to 200 h(-1). Some thermodynamic and kinetic aspects of this biphasic reaction in water/ethyl acetate were studied.