Recent laboratory measurements have shown that the stationary-state vapor pressures of aniline and n-heptanol are enhanced by the application of a positive temperature gradient in the vapor, that the vapor pressure of water over 50 percent sulfuric acid is enhanced by a positive temperature gradient and diminished by a negative temperature gradient, and that values of the Onsager heat of transport derived from these measurements are of similar magnitude to the latent heats of vaporization. When the vapor gap over which the temperature difference is applied is less than about 0.8 mean free paths, the liquid behaves as though its surface were at the temperature close to that of the gas on the other side of the Knudsen layer. These results are discussed in relation to field measurements of the rate of air-sea exchange of carbon dioxide.