The walls of Capo Soprano are one of the most important and well-preserved examples of Greek military architecture. They were built in the second half of 4th century after 339 B.C., in the context of the recolonization of Gela by Timoleonte. Two kinds of building materials were employed. The lower part of the wall (about 2. 80 meter thick) is made of two curtains of calcareous sandstone ashlars with the inner space filled with waste stone materials and earth. The upper part was built with bricks of a material called sun-dried clay , according to a terminology adopted by the archaeologists, but having a composition quite different from that of a true clay. This work, carried out in cooperation with the Superintendence of Archaeological Monuments of Agrigento, deals with chemical and physical characteristics of these building materials. The various morphologies of deterioration as well as the modifications produced in the materials by the natural aging and by the action of water and atmospheric pollutants are studied and discussed, with the aim to ascerain the causes of the decay phenomaena in the monument.