The Whittier Mansion, a four-story residence designed by California architect Edward R. Swain (1852-1902), has been the headquarters of the California Historical Society since 1956. Built of red Arizona sandstone, the building shows advanced decay of the stone (more fragile and faster deteriorating than popular East coast brownstone ). This article summarizes the conclusions of the study to determine the types and causes of the Mansion's exterior decoration, to assess structural soundness, to propose and test alternative conservation treatments and to provide recommendations for stabilization and conservation. -- AATA