In this study, calcium chlorideanodized alumina composites were examined for water vapor sorption properties. Porous alumina host matrices for calcium chloride impregnation were prepared by anodizing aluminum plates in an oxalic acid bath or a sulfuric acid bath. CaCl2alumina composites were obtained through calcination of the anodic alumina film impregnated with CaCl2 solution at 473 or 773 K. XRD analyses indicated that the type of host matrix and the calcination temperature made an effect on crystal states of calcium chloride in the composite layer. The composite calcined at 473 K, of which host matrix was oxalic-acid anodized alumina film, contained calcium chloride anhydrate crystals. Water vapor sorption experiments showed that the composite with crystalline calcium chloride sorbed well water vapor, and its sorption isotherm had a shape similar to that of bulk CaCl2. Therefore, the water sorption behavior of the CaCl2alumina composite strongly depends on crystal states of calcium chloride in anodized alumina layer. IntroductionMetal salts such as calcium chloride, lithium chloride and calcium nitride have a high hygroscopic nature, and their sorption capacities of water vapor are larger than that of silica gel or zeolite. Water vapor sorption on metal salt contains a gas-solid reaction related to the formation of hydrates. According to the Gibbs's phase rule, the reaction between solid metal salt and water vapor is a monovariant system, that is, the sorption equilibrium can be determined by either the vapor pressure or the sorption temperature (Aristov, 2007). This property is desirable as working pair for sorption chillers. Many researchers have proposed to immobilize metal salts in porous materials such as silica gel (Aristov et al., 1996a), alumina (Sharonov and and expanded graphite (Fujioka et al., 2008) to ensure easy handling of the metal-salt particles and/or to enhance the heat and mass transfer rates in solid-sorbent beds.Aluminum is one of the most suitable materials for heat exchangers because of the high thermal conductance. If aluminum plates have the hygroscopic properties, they will be very attractive sorption materials that can be rapidly cooled and heated. In order to meet this requirement, we prepared aluminum plates with porous thin aluminum oxide films on both sides by anodizing in acidic baths, and then calcium chloride was deposited into pores of the films by a solution impregnation method in the previous works (Kumita