Oxidation of niobium at 700 degreesC and zirconium at 850 degreesC. in air or oxygen pressures from 20 to 1 000 hPa, lead to the formation in one case of a stratified Nb2O5 oxide scale and in the other case a more compact ZrO2 oxide scale. Analyses of the evolutions of phases, crystalline textures and mechanical stresses during oxidation are determined by X-ray diffraction techniques. Interactions between chemical processes, crystalline building and mechanical processes constitute together the driving force of the reaction and can explain the differences in morphologies of oxide scales.