The heterogeneous catalytic hydrogenation of polystyrene (PS) dissolved in decahydronaphthalene was studied in a batch reactor using 5% Pd/BaSO4 as the catalyst. The effects of temperature and H-2 partial pressure were investigated over the ranges from 90 to 180 degreesC and 1.72 to 6.89 MPa. The PS concentration ranged from 1 to 9 wt %. The rate of ring hydrogenation was approximately first-order with respect to PS, with an apparent activation energy of 59.6 kJ/mol. The hydrogenation rate depended on the agitation rate up to a threshold value, which increased with the concentration of PS in the solution. The possible influence of three mass-transfer steps (H2 transport from the gas into the polystyrene solution, PS transport to the surface of the catalyst particles, and PS diffusion in the pores of the catalyst) is analyzed. The last two steps do not appear to be kinetically important at the conditions of this study.