The response times of optical emission spectroscopy (OES) and mass spectrometry (MS) have been measured for plasma etching of III-V heterostructures. For the Ga optical emission signal at 417.2 nm, a response time as fast as 0.2 s was obtained. The minimum response time of the (AsCl2+)-As-145 partial pressure, measured by MS, was found to be 0.9 s. The saturation times of the optical emission signal and the partial pressure signal have been shown to be related to the residence time of etch gases by varying the total gas flow rate and the chamber pressure. Decreasing residence time by reducing the pressure from 6 to 2 mTorr and maintaining a constant flow rate caused the saturation time of the Ga emission signal at 417.2 nm to decrease from 7 to 3 min. The (AsCl2+)-As-145 partial pressure signal saturated before the Ga emission signal. Endpoint detection for etching an AlInAs emitter and stopping on a GaInAs base of a heterojunction bipolar transistor was studied. Algorithms which monitor the change in Ga emission intensity have been developed to automatically stop the emitter etch with similar to 2 nm of the GaInAs base layer removed. Additionally, etching of GaInAs on an InP substrate was studied and the signal from OES detected the endpoint before the MS signal did due to the faster response time.