BACKGROUND: Catalytic hydrogenation is gaining interest as an emerging technology for the detoxification of industrial effluents containing pollutants such as nitrophenols. This technique achieves high conversion levels under ambient-like conditions of temperature and pressure, leading to less harmful products than the starting compounds. RESULTS: Complete conversion of 2-nitrophenol, 4-nitrophenol and 2,4-dinitrophenol was achieved in short reaction times at 25 degrees C and 1 atm, using Pd/Al2O3 and Rh/Al2O3 catalysts. The rates of disappearance followed the sequence: 2,4-dinitrophenol > 2-nitrophenol > 4-nitrophenol, the Pd catalyst being more active than the Rh one. The corresponding amino-compounds were identified as the only reaction products. From a Microtox test the reaction products appeared more toxic than the starting compounds. Nevertheless, respirometric tests with conventional activated sludge yielded EC50 values for the aminophenols about double those of the nitrophenols. Biodegradability tests showed that the monoaminophenols can be degraded by non-acclimated sludge, whereas no degradation was observed for the nitrophenols and 2-amino-4-nitrophenol, the reaction product from 2,4-dinitrophenol hydrogenation. CONCLUSION: Catalytic hydrogenation of wastewater containing nitrophenols gives rise to byproducts less toxic and more easily biodegradable by an activated sludge. Copyright (c) 2012 Society of Chemical Industry