In this work we present the investigations on the determination of the mechanism of sodium loss in High Pressure Sodium (HPS) lamps. The transport through the ceramic arc tube consists of several steps (solution in the ceramics, diffusion through the ceramics, leaving the bulk phase, evaporation from the surface). Among the listed processes the mechanism of the diffusion was investigated by model-experiments. A ceramic arc tube was applied as a reactor chamber, in which polycrystalline ceramic plates were heated at 600-1100°C in controlled Na and Xe atmosphere. The sodium diffusion profiles developed by the heat-treatment were measured by XPS and SIMS and from the measurements we tried to draw a conclusion about the mechanism of sodium diffusion through the ceramic discharge tube. The ceramic discharge tube were measured by X- ray diffraction, too. We found traces of some Na-O-Al compound.