In a beam apparatus, influence of fluorocarbon gas (C5F8, C4F8, and CF4 molecules) flux onto SiO2 etching reaction is investigated in comparison to a case of CF2 radical flux. Fluorocarbon molecules or CF2 radicals are incident onto SiO2 surface with Ar+ beam of energy 100–900 eV. Atomic composition of SiO2 surface under the ion and molecule incidence is measured by in situ x-ray photoelectron spectroscopy. Fluorocarbon/Ar+ co-incidence enhances the SiO2 etching compared with pure Ar+ incidence, suggesting the etching effect of fluorocarbon molecules under Ar+ bombardment. In the case of C5F8/Ar+ co-incidence, formation of fluorocarbon layer is observed as the case of CF2/Ar+ co-incidence. Etching yields of SiO2 by C5F8/Ar+ and CF2/Ar+ co-incidences are mea- sured for different Ar+ incident energies and fluorocarbon/Ar+ flux ratios. The etching yield increases monotonically with the Ar+ incident energy, and reaches the value of 2.4 at 900 eV in case of C5F8, which is about 1.5 times as high as in the CF2 case. In the case of C5F8 and CF2, a phenomenon like etch stop, with formation of a thick fluorocarbon layer, is observed for larger fluorocarbon flux, with the threshold of the fluorocarbon/Ar+ flux, ratio between 2.5 and 25. These results suggest a significant contribution of direct SiO2 etching by fluorocarbon molecules in practical etching reactors, especially in case of the C5F8 molecule.