Using a parallel acquisition electron energy analyser (the HFA). the presence of sub-monolayer coverage of chlorine on in vacuo cleaved (1 1 0) surfaces of InP and GaAs has been observed. From the time evolution of the Auger features, to,,ether with sample absorption current images and residual gas analysis, it is apparent that both electron stimulated desorption and surface diffusion take place, and that these are influenced by the presence of two adsorption sites of differing strengths. In particular, the sample absorption current images provide a rapid means of measuring the spatial distribution of the chlorine thereby distinguishing the effects of desorption and diffusion. (C) 2002 Elsevier Science B.V. All rights reserved.