The fouling tendency, due to adsorption on the pore walls, of two pairs of modified and unmodified ultrafiltration membranes, with similar observed retentions determined by dextran and gel permeation chromatography, was studied. The membranes investigated were made of modified and unmodified polyaramide (PA) and modified and unmodified polyvinylidene fluoride (PVDF). The PVDF membrane was surface-modified and the PA membrane was made from a modified polymer solution. Membrane modification was used to reduce fouling by adsorption. Octanoic acid was used as the fouling substance, representing a large number of small, hydrophobic compounds. It is demonstrated in this investigation that membrane modification is not always successful. It was determined that at lower concentrations of octanoic acid, the modified PA membrane exhibits a smaller fouling tendency than the unmodified PA membrane, while the result is reversed for concentrations above 60% of the saturation concentration. The fouling tendency of the unmodified PVDF membrane is much lower than that of the modified PVDF membrane at all concentrations. The cross-sections of the membranes were visually examined with scanning electron microscopy, but no difference could be observed between the modified and unmodified membranes. The membranes were also examined with Fourier transform infrared spectroscopy. The spectra of the two PA membranes were different, while no difference was observed for the unmodified and surface-modified PVDF membranes. Remains of octanoic acid were found in the membranes, although they had been thoroughly rinsed with deionized water and the initial pure water flux was recovered.