Gas phase reactions of CFx (x=1-3) radicals and F atoms in an electron cyclotron resonance (ECR) downstream plasma were investigated at 300 W and a CHF3 pressure of 0.4 Pa by adding O-2 to CHF3 using infrared diode laser absorption spectroscopy and actinometry, respectively. By adding a small amount of O-2 to CHF3, the CF and CF2 radical densities rapidly decreased, while the CF3 radical density rapidly increased but then decreased with further addition of O-2 to CHF3. These reaction mechanisms in the plasma were evaluated on the basis of measurement results of CF (x=1-3) radical and F atom densities. Under the same conditions as the measurements of radical densities, the etching rates of Si and SiO2 were measured. Furthermore, the etched Si surfaces were also analyzed with x-ray photoelectron spectroscopy. The etching mechanisms for Si and SiO2 in the CHF3/O-2 ECR plasma were investigated by connecting these etching characteristics on Si and SiO2 with the behaviors of CFx (x=1-3) radical and F atom densities. The Si etching rate was affected primarily by the concentrations of the CF and CF2 radicals and F atoms in the plasma.