The corrosion phenomena still active on the surfaces of the San Marco Horses are complex; they relate to the nature of the metal and to the length of exposure to different atmospheric conditions. This study attempts to present the most important corrosive processes that have taken place and those still in effect. General corrosion processes are first discussed, then the specific corrosion of gilded copper. Maritime conditions prevail in both Constantinople and Venice, but in Venice there is higher humidity and greater rainfall. New chemical reactions are attacking the electrochemical corrosion byproducts; the humidity provides the conditions for attack and transformation of preexisting corrosion products and the progress of galvanic corrosion phenomena; rain washes the surface, resulting in mechanical abrasion.