Graphical correlations of binary gas diffusion coefficients are developed based on the Hirschfelder-Bird-Spotz diffusion equation and the theorem of corresponding states. A critical diffusion coefficient is defined and is used in turn for a definition of a “reduced” coefficient. The reduced diffusion coefficient is correlated graphically in generalized form in terms of the reduced properties of the diffusing gas. Using air as a reference “barrier” gas, the authors compared critical diffusion coefficients for various gases diffusing through a single barrier gas with the critical coefficients for these gases through air. This ratio, termed the barrier gas ratio, was found to be independent of the properties of the diffusing gas. A graphical correlation of the barrier gas ratio enables rapid estimation of a binary diffusion coefficient with a minimum of information.