The shadowing/blocking characteristics of medium energy ion scattering allow a precise deduction of the layer-by-layer composition of alloy crystal surfaces. The technique relies on an accurate alignment of the ion beam with known crystallographic directions to restrict the incoming beam to illuminate just the topmost one, two or three or more layers. Further layer specificity is bestowed by utilising outgoing geometries that block ions from all but the top one, two, etc., layers respectively. The size of shadow cones associated with medium energy hydrogen and helium ions is ideal for this application. A plot of the averaged experimental yields against simulated yields for several different systems implies an inconsistency in the technique. Possible explanations for this are discussed and a clarifying experiment is suggested. (C) 2003 Elsevier Science B.V. All rights reserved.