This study examines the influence of plasma chemistry on SiO2 to Si etch selectivity in high density C2H2F4(1.1,1,2-tetrafluoroethane)/O-2 and CHF3/H-2 discharges. Etch rate measurements of Si and SiO2 have been combined with chemical characterizations of the discharge using optical diagnostics and an in-fine quadrupole mass spectrometer. The gas phase concentrations of CF2 and F as well as the mass spectrum of ions incident at the substrate have been measured for conditions producing SiO2/Si selectivities from 1 to over 30. For low density sources with high neutral ai to ion fluxes, the conventional theory is that selectivity is governed bf the relative content of fluorine versus carbon in the fluorine and fluorocarbon radicals incident on the substrate. However, for the high density plasma discharges studied, the conventional theory may apply only when the contributions of both neutral radicals and fluorocarbon ions, CFx-, are considered. For the case of oxygen addition to C2H2F4 discharges, reactions between the fluorocarbon surface film and oxygen in the gas phase may also be important in controlling selectivity.