Different decay mechanisms, according to the exposure to rain actions, have been identified on a series of Istria stone samples derived from the Venetion buildings. Enquiries have been carried out on these samples by means of porosity measurements and chemical analysis in depth. In order to identify the decay mechanisms and their kinetics, simulation tests have been performed on exposed samples of Istria stone for a period of 19 months. These specimens have been analysed by deveral techniques in order to verify the chemical dissolution of limestone provoked by the rain action. The initial chemical attack on the exposed samples has been pointed out by XPS and RBS micro-analytical techniques. -- CAL