The masking of the hydrolytic precipitation of lanthanum, cerous and ceric ions has been studied by means of the pH titration method. The pH values, under which the solid phases began to separated in the presence of a number of complexing agents, were determined. Based on the experimental data, the sequences of masking ability toward these hydrolytic precipitation reactions were obtained. The relation between the pH at incidence of precipitation and the ratio between the concentration of complexing agent to that of metallic ion was reported. The most effective masking agents for these ions were shown to be pyrocatecholdisulphonic acid and some complexans. On the other hand, hydroxy aridc are poor in mackinv them