The changes in physico-chemical properties of gallosilicate molecular sieves with MFI type structure were studied as a function of thermal treatment in the temperature range 873-1393 K. The percentage relative crystallinity, specific surface area, sorption uptakes, and framework chemical composition of crystalline pentasil gallosilicate were found to be influenced by the temperature and period of heat treatment. At least up to 2 h at 1213 K, no remarkable changes were observed with respect to the stability of the MFI phase and a state of framework gallium. On the basis of severity of heat treatment conditions, the progressive transformation of crystalline Na/GaZSM-5 to cristobalite phase was observed. The nature and concentration of framework cations are also found to influence the rate of transformation of MFI structure into this phase. The correlation between temperature of the phase transformation and the mole ratio SiO 2 /Ga 2 O 3 is systematically studied. The apparent activation energy of transformation of pentasil gallosilicate to cristobalite is 279.6 kJ/mol. The variations in the physico-chemical properties were examined using characterization techniques such as XRD, IR, 29Si MASNMR, SEM, AAS, sorption measurements and wet chemical analysis.