In situ spectroscopic ellipsometry has been used in conjunction with desorption mass spectrometry to determine both the composition of Al and growth rate of AlxGa1-xAs and GaAs at high temperatures (600-720 degrees C). Ex situ cross sectional transmission electron microscopy verified the AlGaAs/GaAs layer thicknesses grown by molecular beam epitaxy at these elevated temperatures. The substrate temperature dependence on Ga desorption rates, given the As, Ga, and Al fluxes and V/III flux ratios, was examined by Monte Carlo simulation. These simulations showed goad agreement with the experimental results.