The increase in reactivity of crystalline V2O5 has been verified from its equimolecular mixture with calcite which was treated at 150°C in a saturated water-vapour atmosphere. Structural and phase changes accompanying the reaction were followed by X-ray diffraction, infrared spectroscopy and scanning electron microscopy studies. The adsorption of water molecules during the process as well as the reaction degree were quantitatively evaluated by thermal analysis. The results showed the formation of calcium metavanadate as a final reaction product and strongly acid gels which were related to V2O5 as intermediate. The crystalline initial V2O5 is decrystallized during the process of adsorbing water molecules with the formation of very acid reactive gels. © 1993.