This discussion deals with the problems associated with wet deposition on calcareous stones and the processes that occur in the presence of water. Two stone deterioration mechanisms are distinguished: chemical by acid pollutants and salt crystallization. Establishing the amount of damage by each is difficult as both may occur simultaneously. As deterioration strongly depends on the microclimate different types of mechanisms; and therefore different rates may be obtained from different areas on the same monument. Photographs, 27 references.