In the present study a membrane sequencing batch reactor (MSBR) was assessed for treatment of petroleum refinery wastewater (PRW). Mean COD, O & G and TPH removal efficiencies of 80%, 82% and 93.4% respectively - achieved during more than three-month operation of the MSBR with real PRW of varying composition - attested to the robustness of the MSBR system for treatment of PRW. The MSBR system was also able to withstand and recover from a toxic shock loading applied during part of its operation. GC/MS analysis revealed that the majority of the organic constituents of the PRW were eliminated as a result of MSBR treatment whereas a minority were only partially biodegraded. The effect of the application of relaxation a hydraulic membrane cleaning method that has been extensively studied in MBRs but not in MSBRs on membrane fouling was both quantified and characterised. The finding of the present work showed that, compared to the use of air scouring alone, relaxation results in a significant drop in the rate of membrane fouling in MSBRS and affects the way mixed liquor physicochemical properties influence membrane fouling. Also, the membrane fouling layer at the end of MSBR operation was visually and chemically characterised.