The investigations on the influence of humic acids (HAs) being representatives of Natural Organic Matter (NOM) and inorganic salts (NaHCO3, Na2HPO4 and Na2SO4) representing inorganic ions in natural waters, on fouling and stability of a polyethersulfone ultrafiltration membrane in a photocatalytic membrane reactor (PMR) are reported. The influence of inorganic ions on the permeate flux was dependent on their concentration and was more significant at higher salts content. The HCO3- ions contributed to the permeate flux decline to the highest extent, which was attributed to both a higher feed pH and a formation of a dense fouling layer. In the presence of SO42- the permeate flux was higher than in case of other ions which was explained in terms of lower thickness of the fouling cake caused by repulsion of TiO2 particles by the membrane under such conditions. The most severe decrease of the permeate flux observed in the presence of a mixture of HAs and inorganic salts was attributed to the formation of a thick fouling cake caused by bridging effect between HAs molecules and TiO2 particles under high ionic strength conditions. The inorganic salts contributed also to a decrease of HAs decomposition rate due to the hole and hydroxyl radicals scavenging effect of the ions. It was found that the conditions prevailing in the PMR contributed to the loss of the membrane separation properties towards dextrans due to the abrasive action of TiO2 particles. However, the extent of the observed decrease in the dextrans rejection was dependent on the feed composition. (C) 2016 Elsevier B.V. All rights reserved.