This paper studies the kinetics of reactions between iron and aluminum oxides in fly ash and sulfuric acid for the production of a complex sulfate-based coagulant. The important part of this research was to monitor the change in the concentration of product during the reaction. The ratio of concentration of ferric sulfate to aluminum sulfate during reaction, and the linearity of plots of a derived concentration of function versus time at different temperatures show that reactions of both iron and aluminum oxides in fly ash with sulfuric acid are second-order with respect to sulfuric acid concentration. Furthermore, the ratio of the concentration of the ferric sulfate to that of aluminum sulfate and the slopes of the derived concentration function versus time were used to obtain the rate constants of reactions. Finally, based on the rate constants obtained at different temperatures, the empirical Arrhenius expressions of iron and aluminum extracting reactions were derived. (C) 2002 Elsevier Science B.V. All rights reserved.