Asymmetric hydrogenation of methyl acetoacetate to methyl (R)-3-hydroxybutyrate by [(R)-RuCl(binap)( p-cymen)] Cl has been studied in methanol-ionic liquid and methanol-dense CO2 solvent systems. The ionic pairs triethylhexylammonium and 1-methylimidazolium with bis(trifluoromethane sulfonyl) imide and hexafluorophosphates were used. The role of ionic pairs on the kinetic parameters and (enantio) selectivity has been demonstrated. Although the CO2 expanded methanol system suffered from a reduction in both reaction rate and product selectivity, this changed in the presence of water. The high selectivity of the optimized methanol-CO2-water-halide system was designed as a consequence of observed additive effects.