The alteration of marble and limestone surfaces of the Reggio Emilia Cathedral has been studied. The impact of air pollution seems to prevail over natural weathering mechanisms, including biological processes. Sulfation is the most widespread and important feature of the damage. The surface transformation of calcite into ankerite is less common. The presence of the calcium oxalates, weddellite and whewellite, in the surface crusts is always associated with an intense proliferation of epilithic and endolithic blue algae and lichens (particularly in the case of marble surfaces) growing between the granules and reaching a depth of 10-15 min; the detachment of granules, presumably due to physical weathering, then becomes more pronounced due to the action of the microorganisms. Since algae and lichens are highly sensitive to industrial pollution, particularly sulfur compounds, this form of deterioration, which was certainly more widespread in the past, is destined to disappear over the next few years. Uricite, a highly insoluble salt, is present in some samples taken from the horizontal surfaces, and originates from the excrement of birds. This mineral is found to be embedded between the salts and does not itself play any role in the deterioration process. However, other organic acids, related to bird excrement, may contribute to an acceleration of the deterioration of the marble and limestone. Finally, the presence of quartz, feldspars (orthoclase, albite), and numerous clay minerals (kaolinte, illite, chlorite, serpentine) in many of the samples is attributed to soil dust, of which they constitute the main components, and are transported into the system by the wind. Although the effects of quartz and feldspars in weathering phenomena appear negligible, it is possible that the clay minerals, with their high cation exchange capacity, may be of some importance in facilitating chemical reactions occurring in the system.