The hydrogenation of poly(styrene) over a Pd/Al2O3 catalyst was studied in reactors where pulsed flows are present due to both extrusion and forced pulsing. The reaction was investigated over a range of flow rates, polymer concentrations, and pulsing conditions. Observed activities were highly related to gas-to-liquid mass transfer rates predicted by a correlation for slug flow in catalyst monoliths. A reactor fed by a liquid-starved extruder is an attractive choice for hydrogenation at low polymer concentrations, where intrinsic reaction rates are approached. Higher polymer concentrations (10 wt %) lead to higher mass transfer resistances and a decrease in observed activity. But in this case forced pulsing has a greater effect on productivity; an optimum pulsing frequency was observed. Selectivity was higher and power input lower than in a stirred tank at comparable conditions. The optimal frequency is higher than those found in previous work on hydrogenations. (c) 2008 American Institute of Chemical Engineers.