Both X-ray reflectometry and X-ray diffractometry techniques are used for the assessment of individual layer thicknesses inside complicated semi-conductor heterostructures, in particular for opto-electronic applications. The use of Fast Fourier transform-based numerical treatments applied to the reflectivity curve allows a fast determination of the individual layer thicknesses. We demonstrate the capability of this method by reporting X-ray reflectometry study on superlattices, multiple quantum wells, and other complicated structures. Typical layer thicknesses from 0.5 nm to 500 nm were successfully investigated. Fast Fourier transform-based procedure has also been employed successfully on high resolution X-ray diffraction spectra. We finally show the complementary of both techniques. (C) 2003 Elsevier B.V. All rights reserved.