A set of experiments was carried out for the aqueous solution of sodium bicarbonate, and also for that of both sodium bicarbonate and sodium arsenite to explain the mechanism of facilitated transport. It was confirmed that the permeation of carbon dioxide is facilitated by the presence of sodium bicarbonate and sodium arsenite. Sodium arsenite, which is well known as a catalyst for the hydration of carbon dioxide, plays a significant role in the facilitation as expected. The liquid membranes used in the experiment were made by soaking the Millipore filters in the solutions of sodium bicarbonate and/or sodium arsenite with known-concentration. A boundary layer approximation has been used to solve the nonlinear governing equations, and the results are in fairly good agreement with experimental data in the most range of sodium bicarbonate concentation except its low concentration.