We report measurements of the reflectance difference (RD) of light polarized along the [110] and [1̄10]directions of a growing (001) GaAs crystal in a molecular‐beam epitaxy (MBE) chamber. Observations of RD transients occurring due to changes in surface stabilization resulting from interruption of the As beam are compared with simultaneous observations of reflection high‐energy electron diffraction intensities and are shown to give both chemical and structural information about the growing surface. Spectral dependencies of the RD data show distinct responses for Ga and Al surface coverages, suggesting optimal wavelengths and polarizations for selective modification of surface dimer bonding during growth.