The effect of seasonal changes, ionic strength, effluent dilution, transmembrane pressure and spiked concentration on the rejection of the pharmaceutical active compounds (PhACs), carbamazepine (CBZ) and diatrizoate (DTZ) by reverse osmosis (RO) and nanofiltration (NF), was studied. They represent two families of compounds owning low/medium hydrophobicity and relatively high stability to biological transformation at wastewater treatment plant conditions, frequently present in effluents. Experiments were performed with membrane bioreactor effluents and ultrapure water (UPW) to test the influence of background matrix on the rejection. Three RO (BW. SW and XLE) and two NF (NF90 and NF270) membranes were tested. RO exhibited high rejection of both the compounds (> 98%) regardless of membrane resolution or conditions tested. The loosest membrane tested, NF270, displayed a marked seasonal influence on CBZ rejection from effluents (up to 92% in summer and down to 50% in winter), whereas its rejection from UPW was steady (74 +/- 7%). These findings strongly suggest that interactions between low/medium hydrophobicity-PhACs and effluent organic matter (EfOM) took place which markedly changed rejection efficiencies. These interactions seem to be highly correlated to EfOM quality, being enhanced during summer, increasing the removal of small PhACs even by loose nonporous membranes, but decreasing during winter, thus reducing rejection. (C) 2011 Elsevier B.V. All rights reserved.