Marble and stone decay processes were extensively studied on several Venetian monuments in the recent past. According to their texture and structure, stone and marble exhibit a different mechanism of decay. A simplified model of stone decay corresponeinf to different morphology of deterioration was obtained by examining samples taken by considering the degree of blackness and the degree of sheltering from rain water. Three types of decay were defined: white washing, dirt accumulation, and dirt washing. Two distinct types of spherical particles were found in the black dendrite-form crusts: carbonaceous particles with a very porous surface, and smooth particles mainly composed of silicon and aluminum. Airborne particles from combustion of fuel are very efficient catalysts of the sulphuric acid formation from sulphur dioxide oxidation. -- CAL