We investigate the relative validity of the Bruggeman effective-medium approximation and several alloy models to describe interfaces in the analysis of spectroscopic ellipsometric data of laminar samples, using data obtained on an AlxGa1 As-x multilayer sample fabricated specifically for this purpose. The investigation highlights the types of errors that result from the use of inappropriate models. Optimum results are obtained with the alloy model where the graded-composition regions are approximated with multilayer stacks. (c) 2008 Elsevier B. V. All rights reserved.