Photoemission spectroscopy with synchrotron radiation was used to study the LaF3/Si (111) interface as a function of annealing temperature for LaF3 films. These films range in thickness from 0.7-11 monolayers and have been deposited at room temperature. The La/Si photoemission intensity ratio at surface sensitive and bulk sensitive experimental conditions shows that island formation occurs at 420-degrees-C. The stoichiometry of the LaF3 films is conserved until desorption begins at approximately 580-degrees-C, where fluorine desorbs from the surface completely and lanthanum remains on the silicon substrate. From a line shape analysis of the Si 2p and La 4d core levels it is apparent that the interface is dominated by La-Si bonds. Changes in the surface Fermi level are induced by the penetration of lanthanum into the surface layers of the silicon substrate upon annealing.