Abstract Alkylation was carried out at room temperature over catalysts prepared by calcination of FeSO4, Fe2O3, and Fe(OH)3 in the air at various temperatures and subsequent exposure to HCl at room temperature for 20 min. The FeSO4–HCl catalysts heat-treated at 700 and 750 °C showed very high activity and selectivity, the other catalysts except for the sulfate treated at 800 °C being almost inactive. The FeSO4(700 °C)–HCl catalyst was found to consist of FeCl3 and α-Fe2O3 by means of Mössbauer effect, the absorption peak areas being 14 and 86% for portions of iron based on the chloride and oxide, respectively. The results of DTA and TGA experiments suggest that the coordinate bond of water molecule on FeCl3 of the FeSO4(700 °C)–HCl catalyst is stronger than that on FeCl3 of the FeSO4 (600, 800, and 900 °C)–HCl and commercial FeCl3. The FeSO4(700 °C)–HCl catalyst showed a much higher activity than pure FeCl3. The iron(III) chloride formed by reaction of HCl and α-Fe2O3 obtained by thermal decomposition of the sulfate at 700 or 750 °C was found to be a very strong Lewis acid catalyst.