Photoreflectance spectra have recently been obtained for a range of single strained Si1−xGexepilayers for 0.12<x<0.24, which are buried under a Si cap. Spectra measured at different positions on a sample wafer showed pronounced changes in the SiGe line shape. Here these changes are shown to be due to phase shifts arising from changing optical interference effects caused by variations in the Si cap thickness. The phase shifts are determined accurately using a Kramers–Kronig analysis and are interpreted in terms of a multiple‐reflection treatment incorporating a calculation of the Seraphin coefficients. This allows the Si cap thiakness changes to be determined and compared to the0results of speatroscopic ellipsometry measurements. ©1995 American Institute of Physics.