To investigate the possibility of fixing CO 2 as carbonates in high temperature formations, hydrothermal reaction experiments were conducted in CO 2-saturated fluid at 200 and 300°C. Numerical computations of phase equilibria based on the experimental results were carried out. The experiments with granite and calc-silicate rock showed that the dissolution of Ca-bearing minerals and the deposition of secondary minerals were encouraged in CO 2-saturated system. According to EDX analysis and numerical phase equilibrium calculations, the secondary minerals formed might be diaspore, kaolinite, clay minerals (such as smectite, muscovite), Ca-alminosilicates (such as clinozoisite, laumontite, prehnite, wairakite) and calcite. However, there was no direct evidence for the formation of calcite. It is implied that the precipitation rate of calcite is relatively slow and the formation of Ca-alminosilicates occur much easily than calcite even if solution is supersaturated with respect to calcite.