Clarification of glycerin-rich solution is required within the oleochemical industry. Ultrafiltration (UF) could be an option for this purpose. However, flux decline during the UF process is a problem since glycerin-rich solution contains fatty acids which tend to condense in the membrane pores. The main objective of the present paper was to contribute towards better understanding of membrane fouling caused by fatty acids and to investigate the effects of membrane hydrophobicity, molecular weight cutoff and temperature on the membrane fouling. Flux reduction was examined for two different types of polymeric membrane materials with different molecular weight cut-offs (MWCO). The membrane materials were polyethersulphone (PES) based and polyvinylidenefluoride (PVDF) based, with MWCOs between 5, 20 and 25 kDa for the PES membranes, and 30 kDa for the PVDF. It was clearly observed that the addition of fatty acids to the glycerin-rich solution caused significant flux decline, even though the molecular weights of water, glycerin and fatty acids are much smaller than the membrane MWCOs. Additionally, experimental results indicated that the more hydrophobic PES membrane was more severely fouled compared to the PVDF membrane due to the chemical nature of the membrane material itself. The relative flux of PES membranes with larger MWCO decreased by up to 33% compared to 5.6% with smaller ones, but did not vary significantly with operating temperature. The surface morphology and the roughness of fresh and fouled membranes were observed using scanning electron microscopy and atomic force microscopy. (C) 2010 Elsevier B.V. All rights reserved.