Corrosion-oxide layers are formed in pressurized water at 633 K on Zr and Zr-1.4% Sn (Zircaloy 4) during 476 and 590 days, respectively. Oxidation is continuously monitored in situ with impedance spectroscopy (IS), and results are compared to ambient temperature impedance. IS confirms at both materials an acceleration of corrosion after the initial phase of parabolic to cubic growth. At Zr-1.4% Sn, accelerated corrosion consists of cycles of little attenuation and about 80 days cycle time following the first cubic period. At unalloyed Zr, where corrosion is slower, acceleration begins after 360 days. IS at Zr-1.4% Sn shows a cyclic evolution/annihilation of one of the two oxide relaxations which traces a bilayer evolution: (i) formation of a protective (sub) layer at the metal/oxide side to a critical thickness, (ii) conductivity increase in that layer during stress relief, and (iii) regrowth of a protective sublayer into the metal. Such behavior is confirmed by scanning electron microscopy micrographs which identify sublayers separated by crack layers. Thickness of the protective layer is estimated from IS. Degradation at the end of each cycle occurs gradually (similar to15 days at Zr-1.4% Sn). Reasons for accelerated corrosion at degraded layers, such as a formation of open porosity soaked with liquid, are discussed. Dielectric and ac conduction loss terms at 633 K dominate impedance up to high frequencies. IS results indicate that higher loss terms in the outer layer reflect solid-state properties of the oxide rather than a liquid soaking of pores or interfacial roughness. (C) 2003 The Electrochemical Society.