The magnitude of electrophoretic mobility (EM) is usually used to represent the repulsion force between colloid particles in an aqueous system. A generally accepted point is that particles will coagulate well when the suspension EM is close to zero. which is often realized by adding trivalent cations such as Fe(III). With regard to Fe(III) coagulation, however, we found that particles were primarily enmeshed by voluminous ferric hydroxide precipitate or flocs, and the enmeshment of particles by flocs was independent of the suspension EM during Fe(III) coagulation. A suspension with either highly negative or positive EM could be well coagulated and clarified if the solution pH and Fe(III) concentration favored the formation and fast growth of ferric hydroxide precipitate. For an effective Fe(III) coagulation, the EM was not necessarily zero. (C) 2009 Elsevier B.V. All rights reserved.