The reliefs were of the main Portal, made of Proconnesian and Lunense marbles, were very badly damaged. A complete examination of the causes of the stone deterioration was carried out prior to restoration intervention. To assess the morphology of deterioration, samples of decayed stone were taken from different parts of the facade and were analyzed by means of scanning electron (SEM) microscopy and EDAX techniques, x-ray diffraction, and liquid ion chromatography. These analyses were compared with a previous survey carried out 15 years ago. The results obtained showed an increase of the process of sulfation probably ascribed to the influence of atmospheric pollutants. For this reason a survey of air pollution measuring sulfur dioxide concentrations was proposed. Areas sheltered from direct rainfall and from surface runs are generally covered by a hard black crust composed of crystals of calcium sulfate dihydrate mixed with various materials of atmospheric origin. Among these the most abundant are black carbonaceous particles, originating from oil firing which are very active as catalysts in the transformation of calcium carbonate into calcium sulfate. The deposition and adhesion of atmospheric pollutants is more efficient on wetted surface far from direct rainfall. The stone in sheltered areas is moistened by the migration of water coming from white areas exposed to the washing action of rainwater. The concentration of sulfates in black scabs taken from sheltered areas is between 50 and 70%. -- AATA