Water sorption of non-glutinous Uruchi and sticky Mochi rice grains of different diameters is kinetically analyzed using linear driving force and autocatalytic models. Water-sorption curves are classified into two types based on their shape and according to the state of rice grains below and above the gelatinization temperature. Below the gelatinization temperature, the apparent activation energy and the frequency factor decrease with increasing rice-particle diameter. The water sorption mechanisms of both types of rice appear to be the same in terms of the enthalpy–entropy compensation effect. Above the gelatinization temperature, it appears that the gelatinizable areas of starch granules contribute to the equilibrium moisture content, whereas the non-gelatinizable areas affect the rate constant.