We report the first use of ellipsometry as a real‐time monitor of III‐V semiconductor crystal growth by molecular beam epitaxy, specifically growth of GaAs and AlGaAs from arsine, triethylgallium, and triethylaluminum sources. Our results provide new insight into the oxide desorption process and show a sensitivity of ±0.03 in compositionsx>0.2 for 10 A˚ thickness increments of AlxGa1−xAs during initial deposition on GaAs.