Natural Organic Matter (NOM) is abundant in natural waters and is one of the major fouling agents during membrane filtration of surface water. This work addresses the evaluation of the influence of the membrane pore size on the permeation characteristics of humic acid solutions. Five cellulose acetate laboratory made membranes were prepared by the phase inversion method covering a wide range of pore diameters 30, 36, 46, 76 and 92 Å. The Ultrafiltration experiments were carried out in flat-cell units of 13.2 cm2. The membrane performance was investigated at transmembrane pressures from 1 to 4 bar and at the values of pH of 5.3 and 7.0. For the tighter membranes CA1, CA2 and CA3 with pore diameters of 30, 36 and 46 Å, respectively, there is a linear variation of the fluxes in the range of pressures up to 4 bar and for both values of pH. A non-linear behaviour of fluxes vs. pressure is displayed at pH 7 by the CA4 membrane with a pore diameter of 76 Å. The same non-linear behaviour is displayed at pH 5.3 and 7 for the CA5 membrane with the highest pore diameter. The fluxes at pH 7 are lower than at pH 5.3 for all the membranes. These experiments show the importance of pore size in the UF performance of humic acid solutions.