We propose a model for a gas-phase etching of silicon dioxide using anhydrous HF gas and methanol (CH3OH) vapor on the basis of its mechanism and characteristics which were reported by previous researchers. Here, the etch reaction rate was assumed to be determined by the formation step of HF2- resulting from the ionization reaction between HF and CH3OH adsorbed physcially on the oxide surface. The validity of this etch model was confirmed by the experimental data obtained from etching thermal oxide and tetraethylorthosilicate (TEOS) oxide at various etching conditions : HF partial pressure of 2 to 35 Torr; CH3OH partial pressures of 0.3 to 4.5 Torr, and wafer temperatures of 25 to 75 degrees C. It was shown that the gas-phase etching of the oxides could be well characterized by the behaviors of physical. adsorptions for HF and CH3OH molecules on the oxides, which were expected from the Brunauer-Emmett-Teller (BET) equation with the values of parameters in the etch model. Also, it was shown that the etch selectivities between the thermal oxide and the TEOS were mainly dependent on the wafer temperature and the reactant partial pressures, and could be in the range of 2 to 30 according to the etching conditions in the gas-phase regime.