Fouling of a bench-scale, hollow-fibre, pressurised ultrafiltration (UF) membrane is described in terms of simple cake resistance theory. The fouling properties of biologically pretreated greywater and sewage matrices are compared with reference to measured values of specific cake resistance and cake compressibility. Results from constant pressure runs between 125 mbar and 1 bar, constant flux runs between 30 and 901 m(-2) h(-1), and constant flux runs with periodic backwashes are also compared. Specific cake resistance values lay between 10(15.5) and 10(16.5) m kg(-1) throughout. It was not possible to distinguish between the values of specific cake resistance for the two matrices, suggesting that the bulk properties of the two cakes are the same. The cakes were highly compressible; specific cake resistance increased by an order of magnitude as the pressure was increased from 125 mbar to 1 bar. The compressibility factor was calculated to be approximately 0.65 for both matrices. Specific cake resistance calculated over short periods between backwashes was found to increase much more rapidly with pressure than for non-backwashed runs. This was accounted for using a simple, empirical permanent fouling model used in conjunction with classical cake filtration theory to provide a relationship for the normalised available area against normalised pressure. (C) 2003 Elsevier B.V. All rights reserved.