The use of ellipsometry as an alternative technique forinsitudetermination of molecular beam epitaxial growth parameters has been demonstrated. Epitaxial growth has been monitored in real time using three discrete wavelengths to extract growth rates and alloy composition. The effect of substrate rotation on the measured growth rates has also been determined by this technique. From measurements of the GaAs growth rates versus substrate temperature, a value of 4.68±0.12 eV for the activation energy for Ga desorption during GaAs growth was obtained. This agrees with values obtained by other measurement techniques.