Aqueous suspensions of calcium carbonate were hydrogenated to yield calcium formate under a gas phase containing both H-2 and CO2. A rhodium(I)-complex of monosulforated tripherylphosphine (mtppms), [RhCl(mtppms)(3)] was used as catalyst. Reaction temperatures were in the range of 20-70degreesC, total pressure 10-100 bar with an optimum p(CO2):p(H-2) ratio of 1:4. Due to the mobile bicarbonate-formate equilibrium. the highest available formate concentration is decreased with increasing temperature but increased with increasing pressure. Interestingly. at 50degreesC and 100 bar total pressure the yield of formate was 143% (based on CaCO3) which implies the formation of free formic acid in the reaction of CO2 from the gas phase. The dependence of the hydrogenation rate on the catalyst and ligand concentrations, as well as that of the decomposition of formate as a function of temperature were also studied. (C) 2004 Elsevier B.V. All rights reserved.