A model was designed to predict the effect of pH and ionic strength on fouling in the cross-flow ultrafiltration (UF)of protein solutions. Ilias and Govind's numerical approach for concentration polarisation was combined with the Stokes-Einstein generalised equation and Bowen and Jenner's osmotic pressure model. Coagulation was predicted when the mass-transport equation diverged to concentrations higher than that of the maximum osmotic pressure. This occurred at concentrations exceeding that of the maximum diffusivity, when diffusion became too weak to resist drag forces. In this case, one or more monolayers of protein deposited. The model was experimentally tested with 1 g/l BSA and Amicon H1P30-20 modules, for a range of pressures, ionic strengths and pH.