In this work, it is studied the influence of the pH of the waste in the coagulation with aluminum by conventional and electrochemical dosing. To do that, the speciation of aluminum as a function of the pH has been characterized, and this information has been used to interpret the results obtained in the treatments (by both coagulation methods) of a synthetic oil-in-water emulsion and an actual effluent of a door-manufacturing factory. Results show that a simple change in the pH of the wastes can result in a significant change in the efficiency of the coagulation process, and that if the same pH conditions are found at the end of the treatment, the efficiencies of the solution-dosing and of the electrochemical dosing technologies are very similar. (C) 2008 Elsevier B.V. All rights reserved.