Abstract The sorption of ammonia/ammonium ion has been studied on crystalline hydrous titanium dioxide fibers with a layer structure, which are prepared from potassium tetratitanate (K2Ti4O9). From the measurement of the distribution coefficient, it was found that this material strongly preferred ammonium ion to sodium ion. The results of the pH titration curves indicated that this material showed a wide range of acidities toward ammonia/ammonium ion and had 2.63 mequiv/g of an apparent capacity which was about a half of the theoretical ion-exchange capacity. The characterizations of the ammonium adsorbed solid phase at various loading levels were carried out by measuring the X-ray powder diffractograms and the water contents. From these studies, it was concluded that the sorption-desorption reaction of ammonia/ammonium ion on this material took place almost reversibly in three stages.