The reaction of Ca(OH)(2) with hydrogen chloride was studied using single crystals to elucidate the reaction mechanism under simulated bag filter conditions. The X-ray diffraction patterns of the chlorinated single crystals indicated that products at both surfaces ((10 (1) over bar0) and (0001) planes on the side and top faces, respectively) were CaCl2 anhydrate and CaOHCl anhydrate. The observed chlorination product layer on the single-crystal surface was only several micrometers thick, even after 24 h. Comparing the thickness of the chlorinated product layer on the (10 (1) over bar0) side face and (0001) top face of the Ca(OH)(2) crystals revealed that the reaction proceeded more rapidly on the former than on the latter. H2O strongly promoted the chlorination reaction.