We have used time-resolved reflection high energy electron diffraction (RHEED) measurements to study the dynamics of a surface, anion exchange reaction. In the experiment, InAs surfaces are exposed to Sb(x) fluxes and subsequent changes in the crystals’ RHEED patterns are examined. We find that when an InAs surface is initially exposed to an Sb flux the specular spot intensity first decreases, then recovers back toward its initial value. The shape of the intensity versus time curves is extremely reproducible if the absolute Sb flux and the Sb species are kept constant. The length of time required for the RHEED pattern to stabilize is much shorter for cracked Sb than for uncracked Sb. The RHEED dynamics are also faster if the total Sb flux increases. The behavior of the RHEED dynamics as a function of Sb flux and Sb species is consistent with the changes in the RHEED pattern being due to an Sb/As exchange reaction on the crystals’ surface. The RHEED data are compared to previously published x-ray photoelectron spectroscopy (XPS) data which studied exchange reactions on InAs surfaces exposed to Sb fluxes. The XPS study confirmed that the incident Sb did indeed exchange with As in the epilayer and estimated the exposure time needed to complete the Sb/As exchange reaction. The time scales for exchange associated with the RHEED and XPS data are in good agreement, which further indicates that the observed RHEED dynamics are due to the Sb/As exchange reaction. Preliminary results from exposing GaSb surfaces to As fluxes show similar RHEED and XPS behavior. This suggests that RHEED could be generally applicable to the study of surface exchange reaction dynamics.