Spectroscopic ellipsometry is used to examine the microstructure of two buried SixGe1-x alloy films grown by molecular beam epitaxy. In each case the optical measurements show these heterostructure materials to differ significantly in composition and thickness from those which were intended. These discrepancies indicate that there is substantial mixing of species across interfaces. Spectroscopic ellipsometry, which in principle could be performed in situ to monitor the growth of such materials, is shown to provide a quick diagnostic of this intermixing.