Differential reflectance (DR), a contactless optical modulation technique, has been used to study GaAs/AlGaAs quantum wells and bulk GaAs, over a wide temperature range (77<T<600 K). The objective of this study was to demonstrate that DR, unlike most optical modulation spectroscopies, can be effectively and effortlessly used at elevated temperatures, and, thus, provide considerable potential forinsitumonitoring. We have used DR to measure the temperature dependence of several critical points of GaAs and AlxGa1−xAs (x=0.31). ©1996 American Institute of Physics.