The steady-state CO2 flux across thin layers of 30 g/100 ml albumin solutions was measured in two different CO2 partial pressure ranges (boundary PCO2 values 3 and 8 torr, and 160 and 650 torr, respectively). From the data the apparent diffusion coefficient for CO2, DCO2, was calculated. In the high PCO2 range a value of DCO2 was found which is to be expected on the basis of diffusion of dissolved CO2 only. In the low PCO2 range DCO2 was about 100% higher than in the high PCO2 range, when carbonic anhydrase was present and the pH was ∼7.7. DCO2 depended on the concentration of carbonic anhydrase. It increased with increasing pH. It is concluded that an additional diffusion of bound CO2 (facilitated CO2 diffusion) occurs in the low PCO2 range and that this transport involves the hydration of CO2. From the diffusion coefficients in the two PCO2 ranges the rate of facilitated diffusion was determined. Approximate calculations show that this rate (at pH ≤ 7.7) can be explained on the basis of the proposed mechanism of facilitated CO2 diffusion: bicarbonate diffusion and simultaneous proton transport by albumin diffusion. The view that facilitated CO2 diffusion is mediated by the diffusion of albumin is supported by the experimental finding of a considerable suppression of the facilitated CO2 flux in the presence of gelatinized agar-agar.