The influence of metals present in fly ash on the dechlorination reaction that occurs at bag filters installed in municipal waste incinerators and melting furnaces has not been investigated previously. Tricalcium aluminate (3CaO center dot Al2O3, C(3)A) is considered to be a precursor of complex oxychlorides, and its reaction with HCl was investigated in a gas flow reactor under simulated bag filter conditions (473 K, 1000 ppm HCl, 30 vol% H2O, 5 vol% O-2). The molar chlorination ratio of C(3)A leveled off at about I after 24 h. The chlorination reaction rate of C(3)A was similar to that of calcium hydroxide (Ca(OH)(2)). C(3)A chlorination products had a mainly amorphous structure. Elemental mapping images of the surface of chlorinated C(3)A indicated the likely generation of complex oxychlorides of calcium and aluminum. It is possible that complex oxychlorides are produced at bag filters from complex oxides generated in high-temperature incinerators.