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.