Knowledge of the neutral gas composition in a discharge is important for understanding the chemical processes involved in both etching and deposition environments. We have performed Fourier-transform infrared absorption spectrometry measurements of CHF3/O2 plasmas in an electron cyclotron etching tool. Spectral bands were observed from both gas-phase and surface-phase species (deposited on the vacuum windows). The primary gas-phase species were CF4, H2O2, and HF. Strong absorption bands due to CFx, where x=1, 2, or 3, deposition on the vacuum windows were also observed. The densities of the gas-phase species were calculated from the strength of the measured absorption. It was found for typical discharge powers that the HF density was approximately 80% of the total gas density and depended on the plasma density and neutral pressure. In addition, because HF, H2O2, and CF4 are not the feed gas, these data clearly show that recycling of the daughter species (C, CFx, Fx, etc.) on the chamber walls play an important role in determining the plasma chemistry in this high-electron density, >1011 cm−3, low neutral pressure, <10 mTorr, discharge.