Fly ash can be used as a filler in polymer composite materials. Two fly ash samples (one from the UK, UKFA, and another one from South Africa, SAFA) were washed with water and HCl and compared to investigate their interaction with a commercial coupling agent employed to enhance their incorporation into the polymer matrix. The removal of certain ions from the filler surface resulted in a decrease in the heat of adsorption values due to the reduced number of linkage points for the coupling agent. The coupling agent-filler interaction was possible for some of the metals of the second group (calcium, magnesium, and sodium) and some of the p group (aluminum, sulfur, and phosphorous) on the surface of fly ash. (c) 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3985-3992, 2013