Hydrogenation of the water-soluble [{RuCl2(mtppms)(2)}(2)] (mtppms = monosulfonated triphenylphosphine) was studied in aqueous solutions in the presence of excess mtppms both with H-2 and with aqueous HCOONa. Depending on the reductant, the pH and H-2 pressure altogether nine hydride species were identified. In acidic solutions at 1 bar H-2 pressure the known [RuHCl(mtppms)(3)] (1) and [{RuHCl-(mtppms)(2)}(2)] (3) were formed, however, elevated pressure led to the formation of trans-[RuH2(mtppms)(4)] (11). In basic solutions at atmospheric pressure cis-fac-[RuH2(H2O)(mtppms)(3)] (12) was observed which was readily replaced by [RuH2(eta(2)-H-2)(mtppms)(3)] (13) at higher H-2 pressures. 13 is the first water-soluble and stable eta(2)-H-2 Ru(II)-complex stabilized only by monodentate phosphine ligands. [RuHBr(mtppms)(3)] (9) and [RuHI(mtppms)(3)] (10) were obtained analogously to 1. In concentrated aqueous HCOONa solutions (often used in H-transfer hydrogenations) the major species was trans-RuH2(HCOO)(mtppms) (14) while in dilute solutions trans-[RuH2(H2O)(mtppms)(3)] (15) could be observed. Formation of these various hydride species offers an explanation for the earlier observed pH and pressure dependence of the rates and selectivities in hydrogenation of unsaturated aldehydes catalyzed by [{RuCl2(mtppms)(2)}(2)] + mtppms.