The poisoning of Pd-carbon (4.1% Pd) catalysts by thiophene, dichloroethane, mercuric chloride and lead, zinc and mercuric acetates at different concentrations (0-5000 g m(-3)) in the liquid phase hydrogenation of o-nitrophenol to o-aminophenol (at 308 K and H-2 pressure of 1508 kPa) in a three-phase stirred slurry reactor has been investigated. The hydrogenation activity of the catalyst is drastically reduced due to the presence of these poisons in the reaction mixture, even at a very low concentration of poison (20 g m(-3)). Among the poisons, mercuric acetate was found to be the most potent.