The increasing atmospheric CO2 level causes global warming and may pose catastrophic effects to the humanity. Among the various options to reduce the CO2 atmospheric loading, hydrothermal reactions may have a high potential for rapidly and effectively converting CO2 into useful chemicals. In this study, the hydrothermal conversion of CO2 into formic acid was carried out by using Fe as a reductant and Ni as a catalyst. The effect of various experimental parameters, e.g., amount of Fe (Ni), Fe/Ni ratio, temperature, reaction time, alkalinity etc. was investigated. Results showed that Ni played a catalytic role in the hydrothermal conversion of CO2 into formic acid. The highest yield of formic acid of 15.6% was achieved under optimal conditions, i.e., Fe/Ni ratio of 1:1, temperature of 300 degrees C, reaction time of 120 min, filling rate of 35% and NaHCO3: Fe of 1:6. Additionally, the selectivity of formic acid was more than 98%. It was also found that the hydrothermal conversion could not occur without either the addition of catalyst or the existence of CO2 when Fe was used as a reductant. The role of CO2 in the hydrogen production was discussed. (C) 2009 Elsevier B.V. All rights reserved.