Infrared laser interferometry is used to measure etch rate, measure wafer temperature, and identify heterostructure layers in situ during reactive ion etching, with or without masked regions. Interference between reflections from the etching wafer surface, buried heterointerfaces, and polished wafer back allows etch rate monitoring and endpoint determination. Changes in the optical path length that occur as a wafer heats and cools upon processing also produce reflected intensity oscillations that allow determination of the process-induced change in wafer temperature. We also show that λ=0.6238 μm light can be used to monitor optically thin heterostructure layers with enhanced depth resolution over infrared light.