AbstractThe peptization of thorium hydroxide and thorium oxide sol on the preparation of microsphere of nuclear fuels are studied. These sols are prepared mainly by dispersing hydrated thorium oxide in nitric acid. A main character of ThO2sol is its NO3−/Th mole ratio. After peptization, the nitrate is presented as two forms; i. e., sorbed by particles of the sol and free in the aqueous solutions. The adsorbed NO3−/Th mole ratio depends on the average crystallite size of thorium oxide. In certain size range, the adsorbed NO3−/Th of ThO2sol prepared from thorium oxalate is inversely proportional to the crystallite size. The different series of thorium oxide are made by different methods. In this paper the crystallite size is obtained from 33 to 84 A by changing calcination tempera