In general, techniques that monitor theinsitugrowth of semiconductor materials with electron diffraction [such as reflection high energy electron diffraction (RHEED)] provide no quantitative information about surface stoichiometry. However, sophisticated materials problems, such as heteroepitaxial systems, require such information. Here we demonstrate that the use of characteristic absences in the fractional order diffraction features in RHEED patterns provide detailed quantitative information for the molecular beam epitaxial growth of materials such as GaAs. It is expected that this approach will lead to the improvement of growth for such interface‐sensitive systems as resonant tunneling devices.