A generalized equation for adsorption of 131I2 gas on geological material was used. Adsorption parameters for typical Japanese geological materials such as granite, tuff, and sandstone are evaluated as a function of temperature and surface area of rock material. Crushed sample investigations have been performed by the column technique at various steps of temperature and gas concentration. The results indicate that the isosteric differential heat of adsorption is in the range of 63 to 92 KJ / mol at 20°C and that the adsorption coefficient depends on temperature and concentration. Water vapor in 131I2 gas also affects the coefficient, which corresponds to the surface roughness of geological material. A generalized equation is derived on the basis of considering the size distribution of geological materials in order to evaluate the adsorption coefficients at various temperatures. By using this equation, the coefficients of intrafault materials in Japanese granite formations are evaluated at ∼103 cm3/g.