This paper studies, by means of X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDXS), the structural changes suffered by amorphous hydrated Nb oxide and mixtures of amorphous and crystalline Nb and Ta oxides subjected to thermal treatments in air and chlorine atmospheres. The air heating of amorphous Nb2O5. nH(2)O leads to different crystalline phases depending on the working temperature. The hexagonal phase of Nb2O5 is obtained at 773 K and the monoclinic phase beta -Nb2O5 is obtained at 1173 K. The thermal treatment of amorphous Nb oxide in chlorine atmosphere decreases the temperature at which phase gamma -Nb2O5 appears and at 1173 K the stable monoclinic phase alpha -Nb2O5 is obtained. Air calcination for 5 hours at temperatures between 973 and 1273 K of different amorphous Nb2O5. nH(2)O - Ta2O5. nH(2)O mixtures does not lead to solid solution of these oxides. Thermal treatment in chlorine atmosphere of amorphous Nb and Ta oxides leads to the formation of Nb2O5 and Ta2O5 orthorhombic solution in one hour at 973 K and in 24 hours at 1223 K, when starting from crystalline oxides. The effect of chlorine is due to the dissolution-recrystallization of the metallic chlorides and the oxygen formed, when the system evolves to chemical equilibrium between solid phases and gaseous chlorine.