Zircaloy-4 has been oxidised in environmental scanning electron microscope (ESEM), at a temperature of 700 °C and the surface of the material has been observed during the oxidation, directly in the ESEM. The material has been subsequently analysed by atomic force microscopy (AFM) and transmission electron microscopy (TEM). Several aspects of the oxidation were revealed in these experiments. Zircaloy-4 oxidises at a different rate depending on the crystallographic orientation of the grains at the surface. The iron and chromium in the secondary phase particles (SPP) diffuse to the surface and oxidise to produce a profile at the surface of the material. The in situ observations reveal no cracks at the surface during the oxidation. Subsequent examination of the surface (ex situ) by AFM reveals the presence of pores in the oxide. TEM examinations indicate the presence of weak internal planes in the oxide layer, inducing cracks parallel to the metal-oxide interface. © 2003 Elsevier B.V. All rights reserved.