The feasibility of using atomic absorption spectrometry (AAS) to monitor aluminum and gallium atoms in a molecular beam of a system designed for growing epitaxial layers of GaAs and AlxGa1−xAs was investigated. Results are reported to demonstrate the effectiveness of the method for monitoring the flux emitted from source ovens during molecular-beam epitaxial deposition. Absorption by atoms was found to be directly correlated with the temperature of the oven and vapor pressure of the metal. No chemical interaction of atomic vapors of Ga, Al, and As was detected in the molecular beam. Atomic absorption spectrometry compared favorably with mass spectrometry for monitoring Ga.