X-ray photoelectron spectroscopy (XPS) and heavy-ion backscattering spectroscopy (HIBS) were used to investigate the chemical states of barium atoms in submonolayer BaF2 depositions on GaAs (100) substrates. XPS measurements showed barium atoms to have two different chemical states at the BaF2/GaAs interface. The relative abundance of these two states was also determined by XPS. The number of barium atoms in each state was found by normalizing integrated XPS peak intensities to HIBS measurements of the total number of barium atoms on the surface. The results were used to formulate a model of a two-stage deposition mechanism whereby BaF2 first reacts with gallium at the surface and dissociates, releasing gaseous GaF. This reaction is apparently self-limiting, resulting in a barium "template" layer that enables subsequent BaF2 molecules to form an epitaxial (100)-oriented film.