Surface photoabsorption (SPA) has been applied to monitor, in real time, the surface of GaAs(100) during chemical dry etching by a molecular beam of HCl. Changes in the HCl flux to the surface at a constant temperature (840 K) have been used to induce changes in the Ga:As ratio on tile surface. These changes in surface stoichiometry have been detected in situ by SPA measurements of the transient fractional change in the reflectance of p-polarized, 488-nm light that is incident onto the surface near the pseudo-Brewster angle. On the basis of results from prior applications of SPA to the study of the atomic layer deposition of GaAs, the changes in the SPA signal as a function of the etching parameters can be correlated with changes in the relative surface densities of Ga and As. The findings are confirmed by independent determinations of the changes in surface stoichiometry made by measuring the time-integrated difference in the fluxes of Ga-and As-containing etching products evolved from the surface as a function of the HCl flux.