Ammonium-ion-exchanged alpha-Zr(HPO4)2.H2O (alpha-ZrP) was obtained as a single phase with the interlayer distance of 9.4 Angstrom by the ion-exchange of proton with ammonium ion. The ammonium ion-exchanged alpha-ZrP could adsorb ill-smelling gases, such as formaldehyde and carboxylic acids (formic acid, acetic acid, propionic acid, and butyric acid). The adsorption amounts of carboxylic acids increased in the order, butyric acid < propionic acid < acetic acid < formic acid, whereas the adsorption amount of formaldehyde was the same as that of butyric acid. It was cleared that the adsorbed formaldehyde was partially decomposed to formic acid and methanol by self oxidation-reduction reaction in the interlayer region as evidenced by solid-state NMR. Thereby the interlayer distance after the adsorption of formaldehyde expanded to 14.4 Angstrom. In the case of formic acid, it was cointercalated into the interlayer region, and the interlayer distance expanded to 11.1 Angstrom. On the other hand, the interlayer distance of the other carboxylic acid-adsorbed compounds decreased to 7.6 Angstrom due to release by the evacuation. (C) 2004 Elsevier Inc. All rights reserved.