We present an application of time-resolved microellipsometry for monitoring of rapid thermal processes (RTP). Microspot ellipsometric measurements in RTP makes possible to use samples of small sizes. We used microsecond-speed ellipsometer based on static division of wavefront polarimeter scheme using no moving elements and signal modulation. The ellipsometer has strain-free microoptics focusing laser beam into 5 mum light spot. The technique was used to study melting of Ge in Si/Ge structures during pulse heating. Irreversible changes in the optical properties of heterostructures were detected, which are induced by quite short anneals (tsimilar to0.1 s) at surprisingly low temperatures (660-740 degreesC). The character of these changes seems to correspond to conversion of Ge layers to alloy GexSi1-x. We tentatively propose that pseudomorphic Ge layers embedded in silicon host experience a momentary transition to the quasiliquid state thus enabling the surrounding silicon atoms to rapidly dissolve, diffuse and resolidify. According to our measurements the lifetime of quasiliquid state is less than 0.25 ms. (C) 2003 Elsevier B.V. All rights reserved.