Scanning tunneling microscopy was used to characterize Br-exposed GaAs(110) surfaces that were heated to 700 K to induce surface etching. Areal analysis of etched surfaces showed that the etch yield (number of substrate atoms removed per adsorbed Br atom) decreased as the initial coverage increased. This reflects competition between reaction channels involving GaBr and GaBr3 evolution which are determined by the local surface Br concentration. A kinetic model demonstrates that the percentage removed by GaBr3 increases with initial coverage but that most of the Ga atoms are removed as GaBr. Arsenic desorbs spontaneously.