The understanding of variation in sieving properties of membranes is of great importance for the successful development of ultrafiltration applications. A Liquid porosimetry technique is presented to quantify the sieving variation among several polyethersulfone ultrafiltration membranes. Observed sieving coefficients were measured with proper precautions taken to control and minimize fouling. These data were translated to intrinsic sieving coefficients using a stagnant film model. The intrinsic membrane sieving coefficient correlated well with the liquid porosimetry data. This liquid porosimetry technique can distinguish between membranes of different molecular weight cut-off and is sensitive enough to capture slight changes in the sieving coefficient of variants of the same cut-off membrane. This technique has several attractive features : it is nondestructive, independent of the module configuration and relatively simple to perform. Two potential applications of this technique are also examined : (1) quantification of the effect of membrane variation on high performance tangential flow filtration (HPTFF) for protein separations and (2) development of a membrane integrity test to ensure batch-to-batch consistency. This technique has the potential for use in membrane quality control, membrane selection, and validation of industrial ultrafiltration processes.