In the frame of a project dealing with the comprehensive study of the corrosion state of the steam generators of the Paks Nuclear Power Plant, Hungary, surface properties (chemical and phase compositions) of the heat exchanger tubes supplied by the power plant were studied by Mössbauer spectroscopy (CEMS), X-ray diffraction (XRD) and X-ray photoelectron spectroscopy (XPS) methods. The work presented in this series provides evidence that chemical decontamination of the steam generators by the AP-CITROX technology does exert a detrimental effect on the chemical composition and structure of the protective oxide film grown-on the inner surfaces of heat exchanger piping. As an undesired consequence of the decontamination technology, a ‘hybrid’ structure of the amorphous and crystalline phases is formed in the outermost surface region (within a range of 11 μm). The constituents of this ‘hybrid’ structure exhibit great mobility into the primary coolant under normal operation of the VVER type reactor.