Oily wastewaters in Tehran refinery has become a big issue of water and environment pollution to be solved urgently. The results of an experimental study on separation of oil from oily wastewaters are presented. A polyacrylonitrile (PAN, 20 kDa) membrane as the ultrafiltration (UF) membrane and an outlet wastewater of API unit of Tehran refinery were employed and the interaction effects of different operating parameters such as transmembrane pressure (TMP), cross-flow velocity (CFV), temperature and pH on permeate flux and fouling resistance were investigated. In this research, Taguchi method (L-9 orthogonal array (OA)) was used initially to plan a minimum number of experiments. In this work, Hermia's models were used to investigate the fouling mechanisms involved in UF of oily wastewaters. The results showed that the best fit to experimental data corresponds to the cake layer formation model followed by the intermediate blocking model for all the experimental conditions tested. A more detailed analysis of the fouling mechanisms was studied via dividing the filtration curves into different regions corresponding to the different fouling mechanisms. Intermediate blocking and cake layer formation or combinations of them were found to occur during the UF experiments depending on the operating conditions. (c) 2009 Elsevier B.V. All rights reserved.