The water-soluble tertiary phosphine complex of ruthenium(II), [RuCl2(PTA)(4)], (PTA. 1,3,5-triaza-7-phosphaadamantane) was used as catalyst precursor for hydrogenation of CO2 and bicarbonate in aqueous solution, in the absence of amine or other additives, under mild conditions. Reaction of:[RuCl2(PTA)(4)] and H-2 (60 bar) gives the hydrides [RuH2(PTA)(4)] (at pH = 12.0) and [RuH(PTA)(4)X] (X = Cl- or H2O) (at pH = 2.0). In presence of excess PTA, formation of the unparalleled cationic pentakis-phosphino species, HRu(PTA)(5), was unambiguously established by H-1 and P-31 NMR measurements. The same hydrides were observed when [Ru(H2O)(6)]tos (tos = toluene-4-sulfonate) reacted with,PTA under Hz,pressure. The rate of CO2 hydrogenation strongly depends on the pH. The highest initial reaction rate (TOF 807.3 h(-1)) was determined for a 10% HCO3-/90% CO2 mixture (pH = 5.86), whereas the reduction was very slow both at low and high pH (CO2 and Na2CO3 solutions, respectively). H-1 and P-31 NMR studies together-with the icinetic measurements suggested that HCO3- was the real substrate and [RuH(PTA)(4)X] the catalytically active hydride species in this reaction. Hydrogenation of HCO3- showed an induction period which:could be ascribed to the slow formation of the catalytically active hydride species.