The influence of pH on the operating conditions of polycation/polyanion mixed ACFs was studied. Water clarification experiments on quartz sand suspensions under varying pH were carried out by using ACFs formed from polyelectrolytes. In the case of an ACF where polycation and polyanion were both strongly ionizable polyelectrolytes, the variation of pH produced no significant effect except for a charge reversal of the suspended solid at the isoelectric point (i.e.p.). Sudden change in the flocculating efficiency takes place at the point where the polycation and the polyanion which are to form an ACF are equal in number of ionizable functional groups (n_c = n_a). In the case of an ACF containing weak polyelectrolyte(s), the situation appeared to be quite different; the pH showed appreciable effects not only at the i.e.p. but also at a point far from the i.e.p. It was found, however, that the results can be explained in quite the same manner as before, if we take into account the variation in degree of dissociation of weak polyelectrolyte(s); the pH where the sudden change in the flocculating efficiency was observed simply corresponds to the point where the polycation and the polyanion are the same in number of really ionized functional groups.