Biological waste treatment systems in refineries may be subjected to oil loadings sufficient to impair treatment efficiencies. Furthermore, deposits of oil scum create nuisances and increase operational problems. Effects of various oil loadings on the performance of bench scale, activated sludge systems were investigated. The study considered oil loadings in the range of 13 to 625 mg/L in the systems. The oil and synthetic wastes were added daily to activated sludge units operating on a batch basis. Analyses included total organic carbon, volatile suspended solids, settleability, and pH.