Using scanning tunneling microscopy, we have studied the structural modifications induced by Cl upon reaction with Si(111)-(7×7). At low coverages, reacted and unreacted sites are distinguishable in both current-voltage curves and topographs. At saturation coverage, annealing produces extensive mass transport in which most of the adatom layer is stripped away and accumulated in pyramidal Si structures, permitting the complete underlying Si rest-atom layer to be imaged. Much of this layer initially exists as nearly-adatom-free Cl-stabilized 7×7 domains, but further annealing converts it slowly to the more favorable bulklike 1×1 structure. Structures intermediate between the 7×7 and 1×1 are observed. © 1990 The American Physical Society.