Thin (similar to 10 nm) BaO films on tungsten substrates, prepared at specific coverages from multilayer Langmuir-Blodgett precursor films, were heated to different temperatures between 200 and 1200 degrees C in a vacuum environment. The chemical changes occurring at the BaO/W interface were characterized by X-ray photoelectron spectroscopy, infrared reflection spectroscopy, spectroscopic ellipsometry, and X-ray diffraction analyses. The results show that a BaWO4 phase forms above an approximate threshold temperature of 800 degrees C and, contrary to previous observations reported for heterogeneous reactions in powders, no evidence for a Ba3WO6 phase is seen. These data form a fundamental basis for understanding the regulation of the emission work function in the operation of an alkaline-earth-coated tungsten cathode fluorescent lamp.