Scanning tunneling microscopy was used to characterize the developing surface morphology found during typical temperature programmed desorption experiments for halogen-GaAs. Surfaces exposed to Br-2 at 300 K were heated to temperatures between 450 and 675 K, followed by scanning at room temperature. This made it possible to relate the temperature-dependent gas phase etch product distribution to the surface structure and thereby examine atomic-level surface processes associated with the evolution of volatile products. We associate the desorption of GaBr3 around 500 K with the initiation of single-layer-deep terrace pits. Desorption of GaBr and As-2 above 600 K accounts for the lateral enlargement of the pits.