The objective was to investigate the aerobic biodegradation of oily sludge generated by a flotation-flocculation unit (FFU) of an oil refinery wastewater treatment plant. Four I m(3) pilot bioreactors with controlled air-flow were filled with FFU sludge mixed with one of the following amendments: sand (M I); matured oil compost (M2); kitchen waste compost (W) and shredded waste wood (W). The variables monitored were: pH, total petroleum hydrocarbons (TPHs), polycyclic aromatic hydrocarbons (PAHs), total carbon (C-tot), total nitrogen (N-tot) and total phosphorus (P,,,,). The reduction of TPH based on mass balance in M1, M2, M3 and M4 after 373 days of treatment was 62, 51, 74 and 49%; the reduction of PAHs was 97%, +13% (increase), 92 and 88%, respectively. The following mechanisms alone or in combination might explain the results: (i) most organics added with amendments biodegrade faster than most petroleum hydrocarbons, resulting in a relative increase in concentration of these recalcitrant contaminants; (ii) some amendments result in increased amounts of TPH and PAHs to be degraded in the mixture; (iii) sorption-desorption mechanisms involving hydrophobic compounds in the organic matrix reduce bioavailability, biodegradability and eventually extractability; (iv) mixture heterogeneity affecting sampling. Total contaminant mass reduction seems to be a better parameter than concentration to assess degradation efficiency in mixtures with high content of biodegradable amendments. (C) 2007 Elsevier B.V. All rights reserved.