Monitoring wafer changesinsituduring plasma treatment provides real‐time feedback for developing and controlling device processing. In this letter we report the use of photoluminescence spectroscopy to monitor epitaxial films of Al0.3Ga0.7As and semi‐insulating GaAs substrates during BCl3plasma etching and H2plasma passivation. Photoluminescence monitoring is used for etching endpoint detection, surface damage quantification, and wafer temperature measurement.