Both the thermal stability and the kinetics of the isothermal decompositions of the selenites of aluminium, gallium and indium have been studied. A relationship between the activation energy of the process and the radius of cations was found. Such a relationship is believed to result from the effect of counter-polarization of the selenite anion. that depends on the cation nature. The negative values of the activation entropy changes show that the active complex has a more complicated structure than the starting reactant. The increase of the observed absolute values of the activation entropy changes from indium to aluminium selenite also indicates that the structure of the active complex improves with the decrease of cation radius. The values of the steric factor calculated from the Arrhenius equation are significantly lower than unity which means that the isothermal decomposition of selenites from IIIB group of the periodic system belongs to the so-called "slow" reactions.