Modification of poly(phthalazinone ether sulfone ketone) (PPESK) by sulfonation with concentrated or fuming sulfuric acid was carried out in order to prepare thermally stable polymers as membrane materials having increased hydrophilicity and potentially improved fouling-resistance. The sulfonated poly(phthalazinone ether sulfone ketone)s (SPPESK) were fabricated into ultrafiltration (UF) and nanofiltration (NF) asymmetric membranes. The effects of SPPESK concentration and the type and concentration of additives in the casting solution on membrane permeation flux and rejection were evaluated by using an orthogonal array experimental design in the separation of polyethyleneglycol (PEG12000 and PEG2000) and Clayton Yellow (CY, MW 695). One UF membrane formulation type had a 98% rejection rate for PEG12000 and a high pure water flux of 867 kg m(-2) h(-1). All the NF membranes made in the present study had rejections of greater than or equal to 96% and one had a high water flux of 160 kg m(-2) h(-1). Several of the NF membrane formulation types had similar to 90% rejection for CY. When the membranes were operated at higher temperatures (80 degreesC), the rejection rates declined slightly and pure water flux was increased more than two-fold. Rejection and, flux values returned to previous values when the membranes were operated at room temperature again. Mono- and divalent salt rejections and fluxes were studied on an additional NF membrane set. Published by Elsevier Science B.V.