Micropollutant removal by membrane filtration is variable and can be influenced by the presence of organic matter. When considering removal mechanisms, many studies have focused on membrane adsorption and solute-foulant interactions; however, little is known regarding the influence of solute-solute interactions as these are typically difficult to quantify. In this study experimental organic matter-water partition coefficients (K-OM) were applied to quantify and elucidate the influence of solute-solute interactions for steroidal hormone removal by ultrafiltration. The results indicated that the removal of all hormones increased in the presence of organic matter and this was related to hormoneorganic matter interactions. Organic matter did not increase membrane adsorption or cause significant fouling for most molecular weight cut-off (MWCO) membranes, thus solute-solute interactions were the dominant mechanism for hormone removal as expected from previous quantification of such interactions using a specifically developed solid-phase microextraction (SPME) technique. While quantification was only partially successful at low organic carbon concentrations, clear evidence of the importance of solute-solute interactions was demonstrated in concentration studies. Experimental removal and estimated removal due to solute-solute interactions for estrone was comparable at high organic matter concentrations of 25-50 mgC/L for both 10 (48-52%) and 100 kDa (33-38%) membranes, suggesting that organic matter concentration was an important factor in solute-solute interactions. This study represents the first time that experimental organic matter-water partition coefficients have been applied to assess solute-solute interactions in membrane filtration, specifically ultrafiltration. (C) 2012 Elsevier B.V. All rights reserved.