Reactive ion etching of III–V heterostructures was monitored using laser interferometry at 632.8 nm and optical emission spectroscopy (OES) between 200 and 800 nm, simultaneously. Three standard plasma chemistries were investigated: (i) CCl2F2/He for the selective etching of GaAs on AlGaAs, (ii) SiCl4/He for the nonselective etching of GaAs/AlGaAs heterostructures, and (iii) CH4/H2for the etching of In‐based III–V compounds. Laser interferometry provided local monitoring of etch rate, independent of etch chemistry. During GaAs and AlGaAs etching, optical emission from etch products was detected, identified, and selected to monitor etching. Using AlCl emission line at 261.4 nm, accurate etch monitoring of GaAs/AlGaAs heterostructures in SiCl4‐based plasma was demonstrated. The high resolution capacity of OES thereby allowed the discrimination of AlGaAs layers as thin as 5 nm. Accurate etch monitoring of InP/In0.53GaAs heterostructures in CH4/H2plasma was also demonstrated using the In atomic emission line at 325.6 nm: detection of In0.53GaAs layers as thin as 3 nm was possible. The combination of laser interferometry and OES was shown to be a suitable tool for III–V device etching.