The chemisorption of chlorine on the Si(111)7*7 surface has been studied using clusters and the well known AM1 semiempirical molecular orbital method. Both the equilibrium structures corresponding to n Cl atoms (1<or=n<or=7) chemisorbing onto the adatom and rest atom positions of the Si(111)7*7 reconstructed surface, and the etching energies for an adatom or rest atom bonded to x Cl atoms (0<or=x<or=4), have been obtained. We find that as successively more chlorine is chemisorbed, an adatom moves from its original threefold site (one chlorine) to an adjacent bridge site (two and three chlorines), and then to a neighbouring on top site (four or more chlorines). A rest atom bonded to up to four chlorine atoms, on the other hand, remains dose to its threefold clean surface position. Our results also predict, in agreement with experiment, that all of the SiClx species (1<or=x<or=4) will occur on an Si(111)7*7 surface exposed to atomic chlorine at low temperature. Increasing the temperature will first result in all of the SiCl3 and SiCl4, and most of the SiCl2, desorbing from the surface. This will be followed by the desorption of the remaining SiCl2, leaving only SiCl on the surface. The final stage of the desorption process involves the removal of the SiCl. Whilst the etching energies appropriate to a rest atom site are generally lower than the corresponding adatom values, the minimum etching energy is found to correspond to the desorption of an SiCl4 complex involving an adatom of the Si(111)7*7 reconstructed surface.