Fly ash was explored as a raw material for the production of a complex coagulant containing ferric sulfate and aluminum sulfate. An analysis for five major metal oxides in 71 fly ash samples collected from 32 power plants located in four different countries showed that the quantity of iron and aluminum oxides, in general, accounted for about 40% of the fly ash. A fly ash sample collected from City Power, Springfield, Illinois (CPSIL), was selected for more complete characterization by means of scanning electron microscopy (SEM) and other techniques. This fly ash was evaluated to determine the efficiency of converting the iron and aluminum components of the material into a complex coagulant when heated with sulfuric acid at different temperatures and reaction times. The maximum concentrations of Fe2+ and Al3+ in the complex coagulant prepared from the fly ash were obtained at 120 degrees C and after 4 h of reaction time. These concentrations were 0.58 M Fe3+ and 0.93 M Al3+, respectively. These concentrations correspond to conversion efficiencies of 84.8% for iron and 55.1% for aluminum in the fly ash. The complex coagulant proved to be an effective agent for reducing the turbidity, arsenic concentration and chemical oxygen demand (COD) of a given wastewater sample. 2005 Elsevier B.V. All rights reserved.