Conventionally treated oil refinery wastewaters still contain about 20 mg/l total hydrocarbons and 30 mg/l suspended solids sloughed from a biological reactor and the temperature is about 35°C. The new European standards will require less than 5 mg/l hydrocarbons and less than 10 mg/l suspended solids. Such standards could be met by an ultrafiltration operation. The M9 Carbosep membrane was selected after this inorganic membrane proved to be a total barrier for the hydrocarbons of a synthetic emulsion made with an Iranian crude oil while giving a high water flux. A systematic study of the influence of the different operational parameters was then effected with a mixed suspension containing hydrocarbons and biological solids sampled from an activated sludge plant. Aggregation processes of hydrocarbons on the bacterial flocs were observed leading to larger particles with an optimal hydrocarbons/biological solids ratio. This induces a significant flux increase up to 150 l/h m2. Progressive fouling can be limited by use of helical baffles introduced in the filtration element operated at 0.5 bar. Experimental data were fitted to a model of cake deposition with retroflux while the steady state results were recalculated in terms of two dimensionless quantities whose experimental values are linearly correlated.