In ionic liqs. (ILs), the activation of a ruthenium(II) catalyst precursor occurs via an unexpected mechanism, which is different from the activation mechanism that takes place in water. The effectiveness of a biphasic catalytic hydrogenation reaction using the ruthenium(II) precatalyst is highly dependent on the nature of both anion and cation of the IL. Evidence is provided that chloride ions are required to generate the active catalyst. Moreover, the influence of the IL cation on the rate of hydrogenation is correlated with the differences in capability of the IL to solvate chloride. A kinetic study provides insights into the mechanism of the formation of the active catalytic species.