Though in question now, recent studies on CO₂ transport across biological membranes have suggested that CO₂ entry into cells can be facilitated by distinct proteins as aquaporin-1 (AQP1) and AE1 the HCO₃^-/Cl^- exchanger. Both may function as a gas pore helping CO₂ transport. To validate the hypothesis that within the same membrane either protein may serve as a gas transfer route for CO₂, RBC ghosts loaded with the proton sensitive dye BCECF were used to monitor intracellular acidification as a measure for CO₂ entry: Compared to the uncatalyzed reaction of CO₂ and H₂O, transient intracellular acidification demanded the catalytic activity of CA as shown with the membrane permeable inhibitor ETX, and it was suppressed by functional inhibitors of AQP1 and AE1 (HgCl₂ & DIDS). Reversal of acidification was initiated by switching from acidifying to inert gas (N₂) creating an outside- directed gradient for CO₂ and protons. With ETX applied prior to the onset of CO₂ intracellular acidification followed the rules of the establishment of the physicochemical equilibrium. Was ETX applied after the onset of CO₂, further increase of the developing outside directed proton gradient was immediately stopped, the overshoot characteristic vanished, and the time course of built up [H⁺] joint that of the non-catalyzed reaction. Ghosts sealed in PBS with and without 5 mM NaHCO₃ differed in the CO₂ entry rate by a negative feedback of about 10%. Acid load by sodium propionate (NaProp) was taken as a CO₂ independent acidifying mechanism. Our results indicate that the cellular process of CO₂ conversion into H⁺ and HCO₃^- is reversible only under the condition of adequate activity of CA, which also determines the elimination of acidic equivalents as reconverted CO₂.