Abstract The effects of 60Co γ-ray irradiation and of heat treatment in air have been studied on the properties of the hydrous titanium oxide ion exchanger. The ion-exchange capacity of the sodium form did not change at all, while that of the hydrogen form decreased a little, by the exposure of 5.2×108 R. The color of the exchanger, initially white, turned yellow upon irradiation. This exchanger was stable against heat treatment up to around 80 °C; thereafter the exchange capacity gradually decreased with the temperature until 324 °C, where it was abruptly reduced to about a quarter of the initial value. The examination of the titration curves showed that the most acidic exchange site was weaker than less acidic sites against heat treatment. The structural formula of this hydrous titanium oxide ion exchanger has been suggested on the basis of the ion-exchange capacity, the thermal decomposition curves, and such auxiliary data as the X-ray diffraction and the infrared spectra.