Ultrafiltration nanocomposite polysulfone (PSU) membranes containing 0 to 5 wt% carboxyl (-COOH) modified multiwalled carbon nanotubes (mMWNT) were fabricated and used to treat oil sands process-affected water (OSPW). Addition of mMWNT to PSU resulted in increased membrane surface hydrophilicity. Small additions of mMWNT (up to 1 wt%) resulted in decreased average pore size on the selective membrane surface. Pure water and OSPW flux followed the same trend, decreasing with small mMWNT additions (up to 0.5 wt%), followed by an increasing flux with higher mMWNT additions. All nanocomposite membranes had a higher rejection of polyethylene glycol (PEG) (76.5%-92.3%) than plain PSU membrane (65.5%). Membrane containing 2 wt% mMWNT had the highest PEG flux and PEG rejection of 83.1%. Addition of mMWNT reduced the tendency for membrane fouling to occur, indicated by a decrease in fouling ratios and increased flux recovery ratios for nanocomposite membranes as compared with those obtained for plain PSU. The dominant fouling mechanisms during OSPW filtration were standard blocking and cake filtration. Additions of 0.2 wt% and 2 wt% mMWNT resulted in permeates with SDI15 values lower than 1.0 and increased acid-extractable fraction removal (11.9% and 13.9%, respectively) as compared with AEF removal by plain PSU membrane.