In this paper we present a first-principle study on the energetics of a single As-2 molecule on GaSb(001) reconstructed surface. In order to shed light into the mechanisms of anion exchange at the Sb-rich GaSb(001) surface, we studied firstly As-2 adsorption and then As for Sb exchange. We identify a surface region where both the processes are energetically favored. The results of this twofold analysis can be combined to derive possible reaction paths for the anion exchange process. (c) 2005 Elsevier B.V. All rights reserved.