The effect of cooling and depressurization on the formation and growth of carbonaceous mesophase in petroleum vacuum residue was investigated using a novel stirred hot-stage reactor. This apparatus allowed the in situ observation of mesophase formation at 440 degrees C and 4-6.5 MPa under both hydrogen and nitrogen atmospheres. The use of a magnetic stirrer enabled experiments with milliliter volumes of liquid and the addition of catalyst. The in situ observations showed that cooling at constant pressure did not change the amount and size of the mesophase domains, except to arrest their growth by stopping the chemical reactions. Depressurization to atmospheric pressure, while maintaining reaction temperature, after the onset of mesophase formation increased the amount of observable mesophase material significantly. Depressurization before the onset of mesophase formation also induced the formation of observable mesophase earlier than if the reactor had been maintained at pressure.