In order to understand better the mechanism of action of a permanent modifier based on iridium deposited on graphite furnaces pretreated by carbide-forming elements (tungsten and zirconium), an XPS investigation has been carried out on samples submitted to thermal treatment simulating the experimental conditions used in electrothermal atomic absorption spectrometry (ETAAS), Platforms pretreated by tungsten and zirconium were analysed: evidence of tungsten carbide and oxides was found in the former whereas mainly oxygenated species were found on zirconium-treated platforms. In both cases the existence of species anchored to graphite substrate via oxygen and chlorine bridges has been hypothesized in order to account for the binding energy values obtained by a curve-fitting process. The deposition of iridium on either tungsten and zirconium-pretreated platforms significantly modifies their surface chemical composition and, in particular, promotes the formation of tungsten and zirconium species at higher oxidation state (for example, tungsten carbide is oxidized to tungsten trioxide), At the same time, a substantial decrease of the overall signal intensities was recorded, most likely due to modifiers removal from the platform surface and/or their migration into the platform subsurface region as a consequence of the thermal treatments required by the platform-modifying procedure. Possible correlations between the chemical composition of surface modifier species that are formed on graphite substrate and previous findings obtained by ETAAS experiments and morphological and elemental investigations have been pointed out. In particular, confirmation has been obtained that in all the modifier systems studied (tungsten, zirconium, tungsten/iridium, zirconium/iridium) there are at least two eventual forms of the modifier at the end of the preatomization treatment, with a prevalence of oxygen-containing species. Thus, differences in thermal stabilization as well as in the ETAAS patterns of analyte atom generation can be ascribed reasonably to the presence of active centres that are chemically different and characterized by different morphologies. Copyright (C) 2000 John Wiley & Sons, Ltd.