Kinetics of the vapor phase hydrogenation of o-chloronitrobenzene (o-CNB) over Pd/Al2O3 catalyst has been studied in a downflow microreactor under atmospheric pressure. Reaction rates have been measured at three different temperatures with respect to the partial pressures of o-CNB and hydrogen. The order of the reaction with respect to o-CNB was 0.53 at 280 degrees C and increased with increasing temperature. However, with respect to hydrogen, a negative order was observed at 280 degrees C, which decreased further with increasing in temperature. The apparent activation energy (Ea), from the Arrhenius plot was found to be 41 kJ/mol. On the basis of kinetic results a surface mechanism is suggested.