Abstract Spectroscopic measurements in the range from 350nm to 1050nm for pure H2O and CO2 fluids at high temperatures and pressures were carried out to observe critical phenomena using an optical cell autoclave. The intensity of transmitted light though the fluid became drastically lower around the critical point in both cases of H2O. The minimum intensity of transmitted light corresponded to the critical temperature and pressure of each fluid. The supercritical region beyond the critical point for water has been inferred to be a homogeneous state, which does not correspond to either a true liquid phase or a true vapor phase. Results of supercritical-state dissolution experiments using granite and quartz show that this fluid can be subdivided into two apparent phases comprising a ‘liquid-like’ region and a ‘vapor-like’ region. Spectroscopic measurements are powerful tool to identify bulk molecular behavior of fluids at sub-and supercritical states.