The characteristics and the causes of decay of stone samples taken from the Arch of Peace in Milan have been examined before undertaking a study of their conservation. Chemical, mineralogical, petrographical, physical and structural analyses have been performed on the samples. The characteristics of the stones have been compared and then correlated with their slate of preservation. The alteration products have also been analyzed and measurements have been taken of the thermohygrometric parameters of the air surrounding the monument (temperature, relative humidity, air pollution, etc.). On the basis of the results obtained, it is possible now to choose the most suitable conservation treatment.