The influence of a small hydrophobic solute (octanoic acid) on the flux reduction and the flux recovery of two ultrafiltration membranes was studied in this investigation. The two membranes were made of polyether sulfone and regenerated cellulose, both with a nominal molecular weight cutoff of 20,000 Da. The concentration of the octanoic acid solution was found to have a significant influence on not only the extent of flux reduction during treatment of the octanoic acid solution, but also on the flux recovery when the membrane was rinsed with deionized water. After treatment of the octanoic acid solution, a pure water flux increase of 1.5 times the pure water flux of the new, unused polyether sulfone membrane was observed. Different causes of the increased pure water flux after treatment of the octanoic acid solution are presented, and the serious consequences this apparent positive effect can cause in industrial applications are discussed.