Patterns and causes of decay of the building stone of the medieval city of Rhodes are studied. The building material is a fossiliferous calcareous sandstone (typical biocalcarenite) which had been examined prior to the beginning of this study. In situ investigation exposed mainly alveolar disease. The development of the alveolar pattern was observed under favorable microclimatic conditions. On monument samples, chemical analysis, electron microscope, and microanalysis were performed. These measurements correlated with the local temperature and humidity changes suggest the main cause of decay to be the action of the soluble salts transferred into the stone pores. Chemical dissolution of the cementing material as a probable cause of decay is excluded due to the absence of the typical products of such an attack. Accelerated aging tests in the laboratory (immersion-drying cycles and salt spray tests) confirm the causes of decay and show the particular role of sodium chloride and sodium sulfate. The electron microscope study of specifically prepared samples shows the development of sodium chloride crystals in the pores, as well as the breakdown of the material. -- AATA