Chemical reaction diversity was examined by hydrothermal experiments using batch and flow through type autoclaves. Dissolution experiments of granite and quartz with pure water up to 600°C and 60 MPa were carried out in order to evaluate chemical reaction under sub-and supercritical conditions. The supercritical region up to critical point for water has been inferred to be a homogeneous state, which conforms to a true liquid phase, nor true vapor phase. In terms of dissolution of granite and quartz, the supercritical state, being artificially defined as higher pressures and temperatures of the critical point, can be subdivided into two apparent phases, comprising a 'liquid-like' region and a 'vapor-like' region. The critical point of various kinds of geofluid, which was composed from solution in H2O-CO2-X (NaCl) system was experimentally determined by visible type autoclave, attached with transparent sapphire windows. Chemical reaction diversity, depending on location of the critical point for a given solution and apparent phase boundary with respect to chemical reaction within the supercritical state, was recognized by hydrothermal experiments.