The flux decline and the gel resistance during the unstirred ultrafiltration of colloidal dispersions of aluminium hydrous oxides were measured at a constant pressure difference of 50-300 kPa and a constant temperature of 293-313 K in a 200-cm(3) batch cell. The conventional cake filtration theory was employed to analyze the flux decline behavior. This simple theory predicts that the J(m)/J ratio rapidly approaches 2, as time approaches infinity. The time at which the gel resistance becomes equal to the membrane resistance ranged from 6 to 12 s under the conditions of our experiments and decreased with the bulk concentration. Thus, the deposited gel layer offered major resistance to permeate flow throughout the whole operation, except in the first few seconds. The specific gel resistance obtained by plotting (t - t(1))/(V - V-1) against (V- V-1)and was compared for two different crystalline structures of aluminium hydrous oxides (boehmite and pseudoboehmite).