Colloidal extractives particles are potential foulants in pulp and paper mill applications. Characterization of them as foulants is a challenging task because of the complexity of these mill waters. In this study one regenerated cellulose (RC) membrane and two polyethersulphone (PES) membranes were used to study the fouling of extractives in ultrafiltration (UF) of two chemithermomechanical (CTMP) pulp mill process waters. The process waters originated from a pulping process using softwood and a mixture of hardwood and softwood. Extractives were analyzed by extracting them from the membranes and further analyzing them with gas chromatography and by measuring Fourier Transform Infrared (FTIR) spectra for the pure, fouled, and extracted membrane samples. In this study retention of extractives was over 90% even though their molar masses are less than 1 kg mol(-1). Both dissolved and colloidal extractives fouled the membranes. The fouling behavior was different for the process waters used and it seems that the individual extractive groups had different fouling mechanisms. Mostly fatty and resin acids fouled the membranes, but sterols contributed to the fouling of RC membrane remarkably even though their amount in the process waters used was low. (C) 2010 Elsevier B.V. All rights reserved.