When oxidation occurs predominantly by cation diffusion the new oxide forms at the free surface. On large surfaces of zero curvature both the oxide and underlying metal should then bee free from stress. Nevertheless, stresses do develop and a widely accepted explanation for this proposes that limited ingress of anions occurs leading to the growth of new oxide at the oxide grain boundaries. It is demonstrated in this paper that this mechanism is untenable for, while the formation of new oxide at an oxide grain boundary can relax an externally applied stress, it cannot create or augment a stress. A more probable explanation for the generation of stress is the formation of new oxide in cracks or open pores in the oxide scale.