Recent studies have evidenced different decomposition behaviour between stored and freshly recrystallized samples of both NaHCO3 and Na2WO4 . 2H(2)O. To verify whether the above findings could be a general property of crystalline materials undergoing thermal decomposition we compared the thermal behaviour of aged and freshly recrystallized samples of a number of crystal hydrates of different nature (e.g. CuSO4 . 5H(2)O, Na3C6H5O7 . 2H(2)O, H2C2O4 . 2H(2)O, Ni(C2H3O2)(2) . 4H(2)O, CoSO4 . 7H(2)O, etc.) The results suggest that the compounds examined can be divided into three main classes namely : i) those for which fresh surfaces retard the decomposition; ii) those for which the decomposition remains unaltered; iii) those for which fresh surfaces enhance the reactivity, It appears that the three classes can be characterized by different kinetic behaviour also. These findings can be interpreted in terms of a three-stage mechanism recently proposed for the dehydration of tetragonal nickel sulphate hexahydrate.