Reactive ion etching processes, currently in use in microstructure fabrication, often use more than one feed gas and more than one reactive etching specie. An example is die use of gases containing both chlorine and fluorine. These processes have been developed empirically with little understanding of the detailed surface science. Directed beam measurements have been carried out using mixed chlorine-fluorine chemistries and an unexpected enhancement of the F-atom etching of Si by molecular chlorine has been observed.