The flux decline behaviour during skim milk ultrafiltration (UF) was investigated at 10 degrees C, 30 degrees C and 50 degrees C. Despite higher fluxes, UF processing at higher temperatures resulted in higher magnitudes and rates of irreversible fouling. Fouling in this temperature range was primarily proteinaceous, consisting of mainly peptides and alpha-lactalbumin (alpha-LA), with minor amounts of beta-lactoglobulin (beta-LG) present only at 50 degrees C. The increase in fouling resistance with processing temperature was concluded to be mainly due to increased pore fouling by alpha-LA deposition, and in part to beta-LG deposition at 50 degrees C. These are attributed respectively to the thermal expansion of membrane pores and reversible conformational changes of beta-LG.