Infrared internal reflection spectroscopy (IR-IRS) and flux decline has been used to examine the build-up of adsorbed foulants on the surfaces of membranes during the treatment of the plug screw feed pressate (PSFP) from the sulfite digestion of wood chips. Of the four commercial membranes studied, hydrophillic cellulose and thin film composite membranes resisted fouling as shown by no drop in flux during in-plant trials and absence of foulants by IR-IRS. In contrast both hydrophobic polyvinylidene fluoride (PVdF) and polysulfone membranes showed rapid flux decline and were found by TR-IRS to be coated with essentially all of the pressate constituents after exposure to the mill effluent. PVdF membranes were examined in detail by IR-IRS to monitor membrane fouling as a function of exposure time in a laboratory permeation test cell and exposure to PSFP in a static contact test. Hydrated lignin sulfonates, as free acid or salts, were found to be the initially absorbed species, with cellulosic oligomers depositing later on the initial foulant layer.