The water-soluble Ru(II)-phosphine complex, [{RuCl2(mTPPMS)(2)}(2)] was found a suitable catalyst for the hydrogenation of NaHCO3 to NaHCO2 in aqueous solution under mild conditions with catalyst turnover frequencies (TOFs) in the range of 35-50 h(-1) at 50degreesC and 10 bar total pressure. The suggested reaction mechanism involves the formation of Ru(II)-hydrides of the general formula [RuHX(mTPPMS)(4)] where X = H-, HCO3- or HCO2-. At 80degreesC and 95 bar total pressure, the reduction of NaHCO3 proceeded with high reaction rate (9600 h(-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 CO2/H-2 gas mixtures. (C) 2003 Elsevier B.V. All rights reserved.