A combined process anaerobic/aerobic in a single reactor was studied. P-nitrophenol (PNP) was used as the model compound. During the anaerobic stage the target compound is modified in such a way that the intermediate product is more readily biodegradable by the aerobic phase. A pilot biofilter was used for the experiment. The biofilter was packed with a volcanic stone (puzolane) to serve as support to the microorganisms and it was inoculated with activated sludge. The microorganisms were acclimated with an initial concentration of 25 mg PNP/l during 75 days (25 cycles). After 230 days of operation, the reaction time of the cycles was reduced to 11.5 h (8 h for the anaerobic phase and 3.5 h for the aerobic one). The PNP was transformed to p-aminophenol (PAP) in the anaerobic phase, with efficiencies near to 100%. A mineralization of 100% of the PAP was found in the oxidative stage. Global efficiencies of PNP mineralization of 98% were obtained. The reaction rates were 16 mg PNP/l-h (PNP uptake), 14 mg PAP/l-h (PAP formation) and 20 mg/l-h (PAP mineralization).