The evolution of the porous network in mesoporous TiO 2 structures during the calcination induced crystallization process has been studied by transmission electron microscopy, small angle X-ray diffraction and N 2 adsorption/desorption techniques. It has been observed that on increasing the calcination temperature, the mesoporous structures undergo phase transition from amorphous to crystalline and their porosity changes from the ordered to the disordered state, generating crystalline fragments or aggregated porous particles. The optimum calcination temperature required for fabricating ordered metal oxide meso-structures using a polymer template has been discussed. The ordered mesoporous TiO 2 with a crystalline framework showed prolonged photocatalytic activity and repeated usage as an industrial photodegradation catalyst, advantageous to the commercial nanocrystalline TiO 2 Degussa P25. Mechanisms of formation and deformation of mesoporous metal oxide crystalline frameworks are proposed.