It is commonly believed that zero suspension electrophoretic mobility (EM) caused by proper aluminum (III) [Al(III)] addition is important for an effective AI(III) coagulation process. Electrostatically stabilized particles with either highly negative or highly positive EM are conventionally believed to be difficult to aggregate with one other because of strong repulsive forces. However, it was observed that an effective Al(III)-coagulation process evaluated by both floc growth rate and particle removal efficiency can be achieved in some cases of highly positive EM or highly negative EM. Similar EM values did not guarantee similar coagulation performance, and different EM values were accompanied by quite parallel coagulation performances in some cases. The results indicate that unlike charge neutralization, enmeshment of particles by voluminous flocs was independent of the EM of particles. Although EM was reversed in sign or was close to zero, the coagulation rate was very slow without flocs, making the particle removal efficiency low. On the other hand, solution pH and AI(III) concentration were found to strongly affect coagulation efficiency. (C) 2008 Elsevier Inc. All rights reserved.