In a comparative research, five polymeric membranes were employed to treat the oily wastewater of Tehran refinery using cross-flow membrane filtration. To do these experiments, the outlet wastewater of the API (American Petroleum Institute) unit of Tehran refinery was used as the feed. Five different membranes were used including two microfiltration (MF) PS (0.1 mu m) and PS (0.2 mu m) membranes and three ultra-filtration (UF) PAN (20 kDa), PS (30 kDa) and PAN (100 kDa) membranes. The performances of these membranes for treatment of the oily wastewater were evaluated and compared. The final permeation flux of oily wastewater was determined for each membrane (76.0, 73.1, 53.7, 32.1 and 96.2 L/m(2) h, respectively). The PAN (20 kDa), PS (30 kDa) and PAN (100 kDa) membranes showed higher rejection, lower permeation flux and higher permeation flux, respectively. The PAN (100 kDa) UF membrane was effective and suitable for treatment of the oily wastewater to achieve up to 94.1%, 31.6%, 96.4%, and 97.2% removal of TSS, TDS, turbidity and oil-grease content, respectively with a permeation flux of 96.2 L/(m(2) h). In this work, Hermia's models were used to investigate the fouling mechanisms of polymeric membranes. The results show that cake filtration model can well predict the initial and final permeation flux of MF and UF membranes. After cake filtration model, the best permeation flux predicted to the experimental data was intermediate pore blocking model and worst predicted permeation flux was for complete pore blocking model. Chemical cleaning of polymeric membranes fouled with oily wastewater was investigated with metal chelating agent (EDTA), surfactant (SDS), and their combination. The results showed that combination of SDS and EDTA are able to clean the fouled polymeric membranes effectively. Mechanisms of the membrane fouling and chemical cleaning of them were also investigated. (C) 2010 Elsevier B.V. All rights reserved.