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This work reports the results of experiments carried out in a sequencing batch reactor (SBR) operated under 24 hour cycles, treating an effluent containing a mixture of oil field wastewater and sewage, in different percentages. The removal of phenols, ammonium and COD was monitored in several experimental runs, varying the dilution degree of the oilfield wastewater (10 to 45 deg. / v/v). The volatile suspendedsolids (VSS) content in the reactor was also monitored and the protein (PTN) and polysaccharide (PS) contents of the suspended biomass were determined. The removal of ammonium and phenols did not vary significantly in the experimental runs, attaining average values of 95% and 65%, respectively. COD removal efficiencies in the range of 30 to 50 deg. / were attained in the experiments carried out with dilution percentages of 45 and 35% (v/v) respectively. An experiment carried out with a lower proportion of produced water (15% v/v), keeping the salinity level corresponding to a higher proportion of industrial effluent (45I v/v), led to an improvement in the COD removal, indicating that the recalcitrance of the organic compounds found in the effluent is the main cause of the moderate COD removal efficiencies attained in the SBR system. With regard to the composition of the microbial flocs, no significant variation was observed in the PS/PTN, PS/VSS and I'TN/VSS ratios when the effluent composition changed (increased salinity and levels of organic material). [ABSTRACT FROM AUTHOR]