We have developed an experimental approach that allows us to quantify unstirred layers around cells suspended in stirred solutions. This technique is applicable to all types of transport measurements and was applied here to the 18O technique used to measure CO2 permeability of red cells . We measure in well-stirred red cell (RBC) suspensions of various viscosities adjusted by adding different amounts of 60 kDa dextran. Plotting vs. viscosity ν gives a linear relation, which can be extrapolated to ν= 0. Theoretical hydrodynamics predicts that extracellular unstirred layers vanish at zero viscosity when stirring is maintained, and thus this extrapolation gives us an estimate of the free from extracellular unstirred layer artifacts. The extrapolated value is found to be 0.16 cm s−1 instead of the experimental value in saline of 0.12 cm s−1 (+30%). This effect corresponds to an unstirred layer thickness of 0.5 μm. In addition, we present a theoretical approach modelling the actual geometrical and physico-chemical conditions of 18O exchange in our experiments. It confirms the role of an extracellular unstirred layer in the determination of . Also, it allows us to quantify the contribution of the so-called intracellular unstirred layer, which results from the fact that in these transport measurements – as in all such measurements in general – the intracellular space is not stirred. The apparent thickness of this intracellular unstirred layer is about 1/4–1/3 of the maximal intracellular diffusion distance, and correction for it results in a true of the RBC membrane of 0.20 cm s−1. Thus, the order of magnitude of this is unaltered compared to our previous reports. Discussion of the available evidence in the light of these results confirms that CO2 channels exist in red cell and other membranes, and that of red cell membranes in the absence of these channels is quite low.