We have investigated the feed gas compositions that result in the fastest etch rates of SiO2 in C2F6/O-2 and C3F8/O-2 based plasmas and have independently measured the radio-frequency electrical characteristics and optical emission spectra of the plasmas. Gas phase Fourier transform infrared spectroscopy was used to quantify the perfluorocompound (PFC) emissions. Under optimal operating conditions the C2F6/O-2 based discharges exhibited significantly faster etch rates than the C3F8/O-2 based discharges. At the gas compositions that resulted in the fastest etch rates (55 mol % O-2/C2F6 and 68 mol % O-2/C3F8) the greatest F atom densities, the most inductive phase angles, and the highest discharge impedance magnitudes were observed. Higher oxygen feed gas concentrations resulted in higher PFC utilization efficiencies and smaller quantities of plasma generated CF4. For identical f'eed gas compositions C2F6/O-2 based discharges produced on average 50% less plasma generated CF4 than C3F8/O-2 based discharges. (C) Air Products and Chemicals, Inc. - 1999. S1099-0062(99)09-072-0.