During the catalytic hydroconversion of vacuum residue fractions of bitumen or petroleum, the formation of mesophase can lead to fouling of reactor internals. In this study we examine the dependence of the onset of mesophase in a batch microreactor designed to enable in situ microscopic observation of the reacting liquid. The following process variables were investigated: temperature, partial pressure of hydrogen, catalyst concentration, agitation speed, and their interactions. Athabasca vacuum residue was converted in the presence of an iron sulfide particulate catalyst, and a statistical model was developed to describe the effects of the process variables on the onset time for mesophase formation. The temperature and catalyst concentration, respectively, had the most effect on mesophase onset time. The time for mesophase formation dropped by 12.1 min when the operating temperature increased from 440 degrees C to 450 degrees C. The mesophase onset time increased by 7.4 min when 3 wt.% of catalyst was added at 440 degrees C, compared to no catalyst addition. Partial pressure of hydrogen was not significant as an independent process variable for the range studied, from 2.4 to 5.2 MPa. The combined effects of hydrogen partial pressure and catalyst concentration were significant, as were the combined effects of temperature and catalyst concentration. (C) 2014 Published by Elsevier B.V.