The increase of the complexity in semiconductor structures raises more and more the need for local evaluation techniques. For example, laser structures with graded-index waveguides are now widespread, but the characterisation of the shape of the gradient is still empirical. In this study, we use transmission electron microscopy (TEM) for the local determination of the chemical composition of GaAs/AlxGa1-xAs semiconductor laser structures, by quantifying the variation of the thickness fringe profiles with the aluminium content on wedge shaped samples. Contrary to former studies, the presented method uses experimental profiles for calibration. The quantification procedure is thus greatly simplified since no simulation program is needed. Measurements can be performed over distances of more than 1 mu m with a spatial resolution of 1 nm, at any depth of the sample. The precision obtained is about +/-0.03 for the aluminium content x, and the results show the potential of the method for the assessment of semiconductor layer structures.