An experimental study is presented of UF and RO membrane fouling by colloidal particles. Membrane flux and rejection are monitored in a laboratory scale, cross-flow, narrow channel unit employing flat sheet membranes, which are challenged by suspensions of mono-disperse colloidal particles. Apart from the influence of basic parameters, such as particle size, concentration, cross-flow velocity aid cans-membrane pressure, the effect of colloid stability is systematically investigated by regulating the suspension ionic strength. Ln parallel, colloid stability is assessed by Brownian aggregation measurements employing light scattering techniques. Finally, the fouling tendency of the model suspensions is assessed by performing standard batch filtration tests for fouling index determination. It is found that stable colloidal suspensions cause relatively less fouling, whereas, when colloid stability is decreased, significant deposits form on the membranes. Furthermore, it appears that the commonly used fouling indices may give opposite trends from those observed in cross-flow filtration tests. Finally, the experimental results are compared with existing colloidal fouling models and it is demonstrated that these models are not adequate to predict membrane performance.