Abstract The cation-exchange behaviour and surface functionality of three different antimonic acids, amorphous, glassy and crystalline, were studied by thermogravimetric measurement, infrared absorption spectrometry and change in uptake of potassium ions upon heating. The ion-exchange capacities of the amorphous and glassy antimonic acids decrease with the loss of chemically bonded water. In contrast, the capacities of the crystalline acid heated within 200°C rarely change because of the loss of the interstitial (free) water and no drastic change occurs in the surface structure of the crystal. The crystalline acid with free water removed adsorbs moisture upon exposure to the atomsphere at room temperature. Upon heating above 300°C, the tenaciously bonded water was removed and the cation-exchange capacity decreased simultaneously. The infrared absorption spectra indicated that the surface functionality of the crystalline acid, similar to that of crystalline sodium antimonate Na[Sb(OH)6], is different from those of the amorphous and glassy acids. It seems that chemically bonded water in the crystalline acid is tenaciously combined as Sb-OH.