Mass transfer analysis in any experimental system is fundamental before studying its kinetics or optimizing a catalyst. The effect of several variables, such as stirring rate, catalyst concentration and particle size, hydrogen pressure, reaction temperature, polystyrene concentration and average molecular weight, both from the experimental and the theoretical point of view, have been studied in the catalytic hydrocracking of polystyrene dissolved in decahydronaphthalene carried out in a stirred tank with hydrogen and catalyst (0.58 wt % Pt/H-beta) in a slurry. The results show that the variables could be adequately selected to prevent both gas liquid and liquid solid mass transfer control. However, severe internal diffusion control occurred as a consequence of polymer molecular dimensions that, in practice, limits the reaction to the outer surface area of the catalyst. Thus, catalyst optimization for this application, and catalytic processes with polymer reactants in general, should be focused on maximizing external surface area.