Rectangular wave current control of the electrochemical reduction of nitrobenzene improves the selectivity for p-aminophenol (PAP) compared to direct current (d.c.) control at the same average current density in a flow-by packed-bed reactor. Optimal increase in PAP selectivity can be obtained at about a frequency of 50 Hz and a duty cycle of 0.2. A mathematical model is set up to incorporate the effects of mass transfer, hydrogen evolution and double layer charging, and is solved using the Duhamel＇s superposition principle and the modified Crank-Nicolson method with the upwind scheme. The conventional d.c. control cases are also calculated for comparison. Calculations can be applied to predict the reaction results under periodic current and d.c. control, but both display the same trends compared to experimental data.