Volatile species produced during CH4/H-2 and CH4/H-2/Ar electron cyclotron resonance (ECR) reactive ion etching (RIE) of InP and GaAs at pressures of 2 mTorr have been investigated with secondary ion mass spectrometry diagnostics. A CH4/H-2 ECR plasma with -120 V rf bias on the sample stage is found to etch InP but not GaAs. However, etching of GaAs does occur at this bias if the CH4/H-2 mixture is diluted with Ar, showing the importance of physical bombardment to "activate" the GaAs surface. Group V hydrides are usually considered the most dominant volatile products in hydrocarbon RIE. In this work, it is shown that Ar dilution of a CH4/H-2 mixture significantly increases the proportion of group V organometallic to hydride species in both GaAs and InP etching. More important, the main group III volatile ions from this etching process have been positively identified for the first time as In(CH3)2+ and Ga(CH3)2+ ions in InP and GaAs etching, respectively. Finally, the application of volatile product identification to end-point detection is demonstrated with a depth resolution better than 50 angstrom.