Experiments have been carried out to vary the stress in the oxide layer during oxidation of Zircaloy-2. This was achieved by varying the thickness of the metal substrate, using a specimen with a tapered wedge-shaped cross-section. X-ray diffraction measurements confirmed that the compressive stress level in the oxide was reduced when the metal substrate was thinner. The rate of oxidation was also slower for conditions where the stress was reduced. The results can be interpreted such that transitions in the growth result from sequential cracking of the oxide when sufficient elastic strain energy accumulates and that the cracks then enhance access of oxygen to the metal interface.