The comparison of Fick's Law for the diffusion of gas i in a vapor to the equations based on the kinetic theory of gases shows that only for certain special conditions is the Fickian diffusion coefficient, DFi, independent of the mole fraction of i and the diffusional fluxes of the other gases. These conditions include the diffusion of a trace concentration of gas through a gas mixture of any composition, equimolar, counter-current diffusion in a binary gas mixture, and diffusion in a ternary mixture where one gas is stagnant. In an O2, CO2, N2 atmosphere, variations in diffusion coefficient of at least 10% from the tracer values are possible with variations in the mole fractions of the gas or the relative magnitude of their fluxes. The temperature and pressure dependency of the diffusion coefficient must also be recognized.