Whilst permeate flux is an important parameter in characterising synthetic membrane performance, it is a poor indicator of surface condition. Membrane pores may be fouled, but the charge of the fouled surface is critical in determining performance. Polyethersulphone (PES) and polysulphone (PSf) ultrafiltration membranes were fouled with spent sulphite liquor and cleaned using sodium hydroxide and Ultrasil 11 over several operating cycles. The Osmonics PSf membrane displayed a greater relative flux decline over several cycles than the Nadir PES membrane. After 15 fouling and cleaning cycles, the relative flux decline for the PSf membrane was 70% and 55% when cleaning with NaOH and Ultrasil 11, respectively. The corresponding relative flux decline figures for the PES membrane after 15 cycles were 45 and 30% for NaOH and Ultrasil 11 cleaning, respectively. Performance and zeta-potential graphs are presented that demonstrate the strong relationship between the fouling and cleaning history, the surface charge and the performance of the membranes in terms of flux recovery. (C) 2003 Elsevier Science B.V. All rights reserved.