The interaction of chlorine with semiconductor surfaces is investigated using the first-principles pseudopotential method. We determine the adsorption geometries of Cl atoms on both GaAs(001) and Si(001) dimerized surfaces in the high-coverage limit. It is found that Cl atoms can break Ga dimers at Ga-terminated GaAs surfaces without any potential barrier. The Cl adsorption simultaneously weakens the Ga-As back bonds at the surface, which contributes to the etching process. The effect of Cl adsorption on the bonding character of semiconductor surfaces is elucidated. © 1993 The American Physical Society.