The creep behaviour of Zircaloy-4 with and without thermally-grown oxide at the surface is studied. It is particularly shown that the creep mechanism is guided by the metallic substrate in both cases: oxide scale is only responsible for a shift of the creep curves. Microstructural observations and acoustic emission are also used to analyze cracking and delamination of the scale in creep. Oxygen diffusion in zirconia is followed using (18)O during creep and we show that bulk diffusion occurs regardless of the presence of cracks. A dependence of oxygen diffusion on applied stress is also found.