Severe flux decline was observed during ultrafiltration of a pulp mill effluent. Membrane fouling was the result of varying combinations of adsorption, pore plugging and concentration polarization or gel layer formation. A wide range of membrane materials and pore sizes were evaluated, showing the relationship between the membrane material, pore size and the relative contribution of the different fouling mechanisms. Individual resistances were evaluated for adsorption, R(a), pore plugging, R(pp), and concentration polarization, R(cp), using a series resistance model. These were based on the pure water flux for(1) the new membrane, J(i), (2) after static adsorption with the mill effluent, J(a), (3) the product rate when ultrafiltering the effluent, J(v), and (4) the pure water permeability with the fouled membrane, J(f). These resistances were shown to be misleading in terms of the observed flux loss for cases with significant adsorptive fouling. Adsorptive fouling was underestimated and concentration polarization overestimated. An alternative method, which we shall call flux loss ratios, is proposed, which is based on the flux decline due to a particular mechanisms as a fraction of the overall flux decline. These new measures more accurately reflect the flux decline attributable to each fouling mechanism.