In this paper, an extended Hertzian approach will be used in order to develop a tool for the analysis of nanoindentation unloading curves of sharp indenters. The approach is based upon the concept of the effectively shaped indenter introduced by Pharr and co-workers. However, in contrast to the Sneddon formulae used by Pharr, the extended Hertzian approach allows a complete evaluation of the elastic field produced by the effective indenter in closed and elementary form and thus a much more thorough discussion of the effects observed during the indentation experiment. As an example the new approach is used here to determine the critical yield stress of a variety of materials of ultra thin coatings directly from the unloading curves of Berkovich indentations. The results are compared with those of other methods and the deviations are discussed. (c) 2005 Elsevier B.V. All rights reserved.