The water-sol. ruthenium(II) complexes [Cp'RuX(PTA)2]Y and [CpRuCl(PPh3)(mPTA)]OTf (Cp' = Cp, Cp*, X = Cl and Y = nil; or X = MeCN and Y = PF6; PTA = 1,3,5-triaza-7-phosphaadamantane; mPTA = 1-methyl-1,3,5-triaza-7-phosphaadamantane) were used as catalyst precursors for the hydrogenation of CO2 and bicarbonate in aq. solns., in the absence of amines or other additives, under relatively mild conditions (100 bar H2, 30-80°C), with moderate activities. Kinetic studies showed that the hydrogenation of HCO3– proceeds without an induction period, and that the rate strongly depends on the pH of the reaction medium. High-pressure multinuclear NMR spectroscopy revealed that the ruthenium(II) chloride precursors are quant. converted into the corresponding hydrides under H2 pressure.