Real time spectroscopic ellipsometry (RTSE) was used to monitor etching of GaAs/Al0.3Ga0.7As/GaAs heterostructures in citric acid:hydrogen peroxide:de-ionized water (25:1:75). Etch rates of GaAs and AlGaAs of 15.3 and 17.6 nm/min, respectively, were determined by numerically fitting the RTSE data. A variable delay in the onset of etching was observed, and was related to variations in surface cleanliness and the initial oxide thickness. Real time monitoring was also used to control stopping of the nonselective etch after removal of the GaAs cap layer. In addition, etch depth control in the AlGaAs layer was demonstrated. Etching was stopped with 100 nm of AlGaAs remaining. Finally, RTSE was used to monitor wet etching of a patterned sample (75% of the surface area was covered by photoresist). © 1999 American Vacuum Society.