Non-destructive analytical techniques employed by a conservator reveal a painted surface beneath Vasari's frescoes on the west wall of the Salone dei 500 in Palazzo Vecchio, Florence. Analysis of the pigments suggests that it is not related to the Vasari fresco over it, that it uses a toothy ground in an oil or resin binder, and contains pigments that can be associated with Leonardo. Art-historical re-examination of documents and descriptions of the Hall supports these suggestions. Misinterpretation of a document established the tradition of locating the Battle of Anghiari on the east wall. Analysis of the building and the documents helps clarify Vasari's description of the Hall and suggests that the seat of the Gonfaloniere and Leonardo's mural were on the west wall. -- AATA