Spatially resolved x-ray secondary electron photoemission spectra revealed lateral changes in quantum efficiency and chemical composition of polycrystalline cesium iodide photodetectors. Such changes depend on the substrate (stainless steel or aluminum). These results emphasize the limitations of conventional, spatially integrated quantum efficiency measurements, and shed new light on the chemical properties of inhomogeneous polycrystalline cesium iodide. (C) 1995 American Vacuum Society.