This paper discusses anew technique for studying membrane fouling. The foulant is introduced into the membrane module in the form of a pulse and the transmembrane pressure (TMP)-time profiles obtained during constant permeate flux ultrafiltration (UF) are used to quantify concentration polarization and fouling. The fouling of a 25 kDa polysulfone membrane by BSA is studied at different constant permeate flux values at conditions where there is total rejection of the protein by the membrane. Fouling is found to take place in two distinct phases: (i) an initial rapid fouling phase; and (ii) a linear fouling phase. The effect of the permeate flux history of a membrane on fouling is also examined.