Utilizing infrared diode-laser absorption spectroscopy (IRDLAS) and UV-Visible absorption spectroscopy (UV-Vis), we show that it is possible to make a near complete mass balance of etch reactants and products in a GEC inductively coupled fluorocarbon discharge while actively etching SiO2 substrates. Langmuir probe measurements were performed to measure the total ion current density. C2F4 and CF2 are shown to be the main dissociation products in a C4F8 plasma discharge. The C2F4 concentration decreases as the SiO2 etching rate increases, along with CF2 and CF radicals, suggesting a role in the SiO2 etching process. The addition of At to the C4F8 discharge increased the ion flux at the wafer surface, and the consumption rate of C2F4 relative to CF2. The increased ion flux enhanced the SiO2 etching rate, until at a very high degree of Ar dilution of C4F8/Ar the etching rate became neutral limited. We also monitored SiF2 using UV-Vis absorption and CO by IRDLAS. In our work we found SiF2 and CO to be the prevalent Si and C gas phase etch products for the SiO2 etching process. © 2005 American Vacuum Society.