The effect of the molar ratio of hydroxide and fluoride ions to Al(III) ions (gamma(OH) and gamma(F)) on coagulation and electrocoagulation (EC) was studied to solve the problem of the over addition of acid or base. The efficiency of defluoridation was approximately 100% when the sum of gamma(OH) and gamma(F) (gamma(OH+F)) was close to 3. This finding reveals that the fluoride ions and the hydroxide ions can co-precipitate with Al(III) ions and the formula of the precipitate is AlnFm(OH)(3n - m). However, when gamma(OH) was less than 2.4, the defluoridation efficiency, given that gamma(OH+F) = 3, dropped as gamma(OH) fell, because the amount of aluminum polymer formed dropped. The efficiency of defluoridation of EC exceeded that of coagulation for equal gamma(OH) and gamma(OH+F), when gamma(OH+F) > 3, proving the existence of an electrocondensation effect. (C) 2004 Elsevier Inc. All rights reserved.