Under particular temperature and gas conditions the reactivity of the Pd(1 0 0) surface toward CO oxidation exhibits oscillatory behaviour. Here we examine the surface structure of this model catalyst and show that the periodic pattern is more complex than previously reported and that superimposed on the overall oscillation much faster structural variations are present. By examining the structure of the sample surface at high temporal resolution we conclude that the structure of the oxide layer present at the surface evolves continuously toward a more disordered phase in agreement with the Mars-Van Krevelen reaction mechanism.