A membrane separation process, nanofiltration (NF), has been applied successfully for treatment of oil sandswaters, particularly to water softening and removal of toxic components. This study focused on the selection of appropriate membranes and the assessment of their performance for the removal of polyvalent ions (hardness) and naphthenic acids (NA) (the main acute toxicant in oil sands process-affected imported waters) from both imported and potential discharge waters. Experiments were carried out using a bench-scale flat sheet membrane system with several commercially available NF membranes. It was found that after membrane filtration, both water hardness and the NA concentrations were reduced significantly (>95%). A permeate flux was maintained at 15 L/m2/h or higher, with a retentate volume of about 10% of the feed volume.