This chapter contains sections titled: Motion of Molecules and Diffusion Diffusion Phenomena Definition of Diffusional Flux and Reference Velocity of Diffusion Binary Diffusion Flux Diffusion Coefficients Binary Diffusion Coefficients in the Gas Phase Multicomponent Diffusion Coefficients in the Gas Phase Example 2.1 Solution Rates of Mass Transfer Definition of Mass Flux Unidirectional Diffusion in Binary Mass Transfer Equimolal Counterdiffusion Convective Mass Flux for Mass Transfer in a Mixture of Vapors Example 2.2 Solution Mass Transfer Coefficients Example 2.3 Solution Overall Mass Transfer Coefficients References