The advantages of in-situ SIMS plasma probe diagnostics are highlighted in low pressure hydrocarbon ECR reactive ion etching (RIE) of III-V materials. Three aspects of the RIE process are investigated. First, the dominant ion species in a CH4/H2/Ar plasma are recorded at various chamber pressures, ECR powers, CH4/(CH4+H2) gas flow ratios and microwave cavity tuning. These studies have improved our understanding of the effects of these parameters on the relative concentrations of reactive precursor species in the plasma and have led to more rapid optimization of the etch system. Secondly, SIMS has been used for identification of reaction products from the III-V surface at the optimized plasma conditions. The Ar diluted mixture gives rise to significant levels of group V hydrides and organometallic compounds and the dominant group III volatile ions have been positively identified as dimethyl species. The third and final aspect reported is the application of volatile product identification to endpoint detection. In lcm2 multiple quantum well samples, layers as thin as 50Å are easily distinguishable.