This report presents techniques for characterizing Si/Si1-xGex multiple-quantum-well (MQW) structures, during their growth on Si(1 0 0), by means of in-situ spectroscopic ellipsometry. The ellipsometric angles (psi and Delta) exhibit short-period oscillation over time, and this reflects the alternation of the top surface, between Si and Si1-xGex. At photon energies to which the materials are transparent, a long-period modulation structure is also superimposed on the curve as an envelope. This is a result of optical interference between the MQW as a whole and the Si substrate. These features are useful for monitoring the number of layers, the Ge content, and the growth rate. Maintenance of the two-dimensionality of the MQW structure can be confirmed by a glance at the psi-Delta plot at 3.4 eV; the trajectory path soon settles into a closed loop of finite size, with the region of lower Delta corresponding to a top layer of Si1-xGex and the region of higher Delta to a top layer of Si. (C) 2002 Elsevier Science B.V. All rights reserved.