The water-soluble Ru(II)-phosphine complex, [{RuCl 2 (mTPPMS) 2 } 2 ] was found a suitable catalyst for the hydrogenation of NaHCO 3 to NaHCO 2 in aqueous solution under mild conditions with catalyst turnover frequencies (TOFs) in the range of 35-50h - 1 at 50 o C and 10bar total pressure. The suggested reaction mechanism involves the formation of Ru(II)-hydrides of the general formula [RuHX(mTPPMS) 4 ] where X=H - , HCO 3 - or HCO 2 - . At 80 o C and 95bar total pressure, the reduction of NaHCO 3 proceeded with high reaction rate (9600h - 1 ) hitherto unobserved in purely aqueous solutions. The reactions do not require the presence of organic amine additives, however, the addition of quinoline increased the rate considerably. Aqueous suspensions of calcium carbonate could also be hydrogenated with CO 2 /H 2 gas mixtures.