High-resolution X-ray diffraction rocking curve (RC) and X-ray reflectivity (XRR) were used to characterize the Si based heterostructures grown by reduced pressure chemical vapour deposition. The investigation focused on the reliability and accuracy of thickness measurement by the different techniques. For smooth Si epilayers grown on a thin (20 nm) strained Si0.9Ge0.1 buffer, it is found that both XRR and RC produce reliable values that agree well with transmission electron microscope (TEM) results over a wide range. The best-fit thickness from both XRR and RC is within +/- 5% of the TEM measurement, with XRR producing more accurate values than RC. However, the agreement is not good for Si epilayer grown on a thick (2 mu m) relaxed Si07Ge0.3 virtual substrate due to the presence of rough surface. (C) 2011 Elsevier Ltd. All rights reserved.