Using simultaneous nitric oxide and carbon monoxide lung transfer measurements (TLNO and TLCO), the membrane transfer capacity (Dm) and capillary lung volume (Vc) as well as the dimensionless ratio TLNO/TLCO can be calculated. The significance of this ratio is yet unclear. Theoretically, the TLNO/TLCO ratio should be inversely related to the product of both lung alveolar capillary membrane (µ) and blood sheet thicknesses (K). NO and CO transfers were measured in healthy subjects in various conditions likely to be associated with changes in K and/or µ. Experimentally, deflation of the lung from 7.4 to 4.8L decreased the TLNO/TLCO ratio from 4.9 to 4.2 (n=25) which was consistent mainly with a thickening of the blood sheet. Compared to continuous negative pressure breathing, continuous positive pressure breathing increased this ratio suggesting a thinning of the capillary sheet. It was also observed with 12 healthy subjects that slight haemodilution which may thicken the blood sheet decreased the TLNO/TLCO ratio from 4.85 to 4.52. In conclusion, the TLNO/TLCO ratio is related to the thickness of the alveolar blood barrier. This ratio provides novel information to the analysis of the diffusion property.