The mechanisms of facilitated transport was studied experimentally with aqueous solutions of sodium bicarbonate and sodium bicarbonate-sodium arsenite. Both these salts facilitated the permeation of carbon dioxide. Also, as expected, sodium arsenite, a well-known catalyst for the hydration of carbon dioxide, played a significant role in facilitating permeation. The liquid membranes were made by soaking milipore filters in aqueous solutions of known concentration. The solution of the basic nonlinear equations by a boundary-layer approximation agreed closely with experimental data over the entire range of sodium bicarbonate concentrations except at low concentrations.