An in situ cylindrical internal reflection infrared technique (CIR-FTIR) was developed, which permits the real time analysis of supercritical fluids and heterogeneous catalytic processes at temperatures up to 500 degrees C and 1000 psi pressure (1 psi = 6850 Pa). High-quality spectra were obtained at both high temperatures and high pressures under in situ reaction conditions. The molecular thermal transitions that a hydrocarbon undergoes in the supercritical regime, the properties of a hydrocarbon within the pores of a zeolite, and the interactions of a hydrocarbon with the active acid sites of the zeolite during catalytic cracking were studied by this technique. The results showed that the stretching frequency of the C-H bonds was altered in supercritical heptane, probably due to intermolecular hydrogen bonding. IR data also demonstrated an increased heptane concentration within the micropores of a commercial catalytic cracking, Y-type zeolite (promoted Octacat) during catalytic cracking at supercritical conditions. This method also enabled a determination of the types of hydroxyl groups contained within the zeolite (i.e., Bnonsted acid sites in the supercages and sodalite cages, terminal silanols, and superacid sites) and their relative concentration changes with increasing temperature. Finally, the alteration of the concentrations of the various catalytic active sites together with the appearance of new bands was also monitored in situ during catalytic cracking of heptane at 475 degrees C under subcritical and supercritical conditions.