Aerosol samples consisting of fission products and elements of light water reactor structural materials were collected during laboratory-scale simulation of the heat-up phase of a core melt accident. The aerosol particles were formed in a steam atmosphere at temperatures of the melting charge between 1200 and 1900°C. The investigation of the samples by use of x-ray photoelectron spectroscopy (XPS) permitted the chemical speciation of the detected aerosol constituents silver, cadmium, indium, tellurium, iodine, and cesium. A comparison of the elemental analysis results obtained from XPS with those achieved from electron probe x-ray microanalysis revealed that aerosol particle surface and aerosol particle bulk are principally composed of the same elements. The compositions determined in dependence of the release temperature reflect the differing volatilities of the detected elements. Quantitative differences between the composition of surface and bulk have been observed only for those aerosol samples that were collected at higher melting charge temperatures. These samples show an enrichment of more volatile species at the particles’ surfaces. In order to obtain direct information on chemical species below the surface, selected samples were argonion bombarded. Changes in composition and chemistry were monitored by XPS, and the results were interpreted under consideration of possible influences of the sputter process on the surface composition.