Thermokinetics of heat evolution during the rehydration of transition aluminas using an isothermal differential microcalorimeter is presented. The transition aluminas have been prepared by flash calcination of gibbsite. The results obtained are discussed in relation to phase transitions connected with the crystallization of amorphous aluminum gels that are formed during the contact of water with the surface of transition aluminas. During rehydration they transform into more orderly pseudoboehmite gels. The final stage of the hydration, especially at pH greater than or equal to 10, is crystalline Al(OH)(3), mainly bayerite.