The early stages of passivation in high temperature water of a nickel-chromium-iron alloy (Alloy 600) have been investigated by X-ray Photoelectron Spectroscopy (XPS) and Scanning Tunneling Microscopy (STM). The samples (polycrystal Ni-16Cr-9Fe (wt. %) and single crystal Ni-17Cr-7Fe (1 1 1)) have been exposed for short time periods (0.4-8.2 min) to high temperature (325 degreesC) and high pressure water, under controlled hydrogen pressure, in a microautoclave designed to transfer the samples from and to the XPS spectrometer without air exposure. In the early stages of oxidation of the alloy (0.4-4 min), an ultra-thin oxide layer (about 1 nm) is formed, which consists of chromium oxide (Cr2O3), according to the Cr 2p(3/2) core level spectrum. An outer layer of Cr(OH)(3) with a very small amount of Ni(OH)(2) is also revealed by the Cr 2p(3/2), Ni 2p(3/2), and O 1s core level spectra. At this early stage, there is a temporary blocking of the growth of Cr2O3. For longer exposures (4-8 min), the Cr2O3 inner layer becomes thicker, at the expense of the outer Cr(OH)(3) layer. This implies the transport of Cr and Ni through the oxide layer, and release of Ni2+ in the solution. The structure of the ultra-thin oxide film formed on a single crystal Ni-17Cr-7Fe(1 1 1) alloy was analysed by STM in the constant current mode; STM images reveal that, in the early stages of oxidation, the oxide is crystalline, and the observed structure is consistent with the hexagonal structure of the oxygen sub-lattice in the basal plane (0 0 0 1) of alpha-Cr2O3. (C) 2004 Elsevier Ltd. All rights reserved.