We determine the temporal evolution of the real and imaginary parts of the anisotropic components of the surface dielectric responses Δ(εd) of singular and vicinal GaAs surfaces on interruption of the As flux during otherwise normal growth by molecular‐beam epitaxy (MBE). The measurement of dielectric, rather than simply reflectance, anisotropies is made possible by a new, essentially strain‐free quartz window that not only extends our spectral range but also allows relative phases as well as amplitudes to be measured for near‐normal incidence reflectances along [110] and [1̄10]. Data are obtained in 0.1‐eV increments from 1.5 to 5.5 eV for a singular (001) surface and for vicinal surfaces 6° off (001) toward (111)A and (111)B, respectively. The observed temporal evolutions are approximately piecewise linear or exponential, indicating that each surface passes through several distinct phases before arriving at its final Ga‐stabilized condition. Nearly exponential initial transients with time constants of<1 s are interpreted as evaporation of excess As present during growth. A supersaturation phase transition is observed for the vicinal surface 6° off (001) toward (111)A. Spectra constructed from these transients show optical structures at 1.8–1.9. 2.5, and 4.1 eV that, because they are already well developed at 1 s, can be related to the initial As excess. The change in Δ(εd) that occurs between 2 and 5 s for the singular surface is similar to the Δ(εd) spectrum of Ga–Ga dimers, which is calculated here in a tight‐binding approximation.