Using a compact, retrofittable energy-dispersive detector, we measure the RHEED-induced fluorescence signal at grazing exit during molecular beam epitaxy. First results show sub-monolayer sensitivity to all relevant elements down to Al with a time resolution of 1 s. The method is sensitive enough to detect surface reconstruction changes through relative changes in the As signal. In addition, the background signal shows a variation similar to the surface roughness and independent of the chemical composition. (C) 2002 Elsevier Science B.V. All rights reserved.