A new method for determining binary diffusion coefficients in dilute condensable vapors is outlined. The method is based on condensational droplet growth. It can be used in a large range of pressures and temperatures, even below the freezing point of the condensing vapor. Results are obtained for the mixtures water-nitrogen and water-helium in the pressure range 11–44 bar. The product of pressure and diffusivity is found to be constant. New temperature correlations for the binary diffusion coefficient are proposed, valid for the range 235–375 K. For water-nitrogen the temperature dependence is significantly stronger than predicted by the Fuller correlation; for water-helium, it is weaker.