The Bunsen solubility coeficients, [alpha], in ml/mlXatm, of O2 and CO in water, saline, whole blood, and homogenates of human pulmonary and sheep placental tissue, were determined at 37 C. The method was to equilibrate a sample with O2 and CO in a swirl tonometer and then to extract and measure the dissolved gases manometrically. Values for [alpha],O2, [alpha]CO, and the [alpha]O2/[alpha]CO ratio, inorder, were as follows: water 0.0238, 0.0189, 1.26; saline0.0227, 0.0180, 1.26; blood0.0230, 0.0189, 1.21; pulmonary tissue 0.0213, 0.0179, 1.19; and placental tissue 0.0209, 0.0184,1.13. Results suggest the CO diffuses more rapidly through blood and pulmonary and placental tissue than would be predicted from comparative solubilities of O2 and CO in water. However, differences in tissue solubility were not great enough to explain the larger discrepancies in membrane-diffusing capacities for the 2 gases that were reported previously for the human lung and sheep placenta. The water content in tissue samples could dissolve all the O2 and 94% of the CO that was present In blood, at least 17% of the O2 in solution and 20% of the CO was loosely bound to constituents other than water.