Colloidal fouling is a persistent problem in many membrane processes. In this paper the influence of pH and ionic strength on colloidal fouling was determined under controlled ultrafiltration experiments. The effect of water chemistry can significantly exacerbate colloidal fouling. For example, it was found that by adjusting feed water chemistry from pH 9 to ionic strength = 0.003 M to pH 2 and ionic strength = 0.15 M with all other conditions kept constant increased fouling by approximately 430%. Experimental data suggested that the colloidal fouling potential was linearly related to the zeta potential (zeta) of the colloids in the feed water and the natural logarithm of feed water ionic strength or the colloidal double layer thickness (1/kappa). These findings were especially significant because it linked fundamental electrokinetic or feed water properties directly to colloidal fouling in membranes. (c) 2007 Elsevier B.V. All rights reserved.