Abstract –The rate of interpenetration of two initially separated gases of appreciably different molecular weight is studied analytically and numerically. The gases are confined between two infinite parallel impervious planes situated perpendicular to the gravitational acceleration; the lighter gas is initially confined to a horizontal layer,which lies (a) adjacent to the upper plate, and (b) adjacent to the lower plate. The initial stratification in (a) is relaxed by Fickian diffusion, with barodiffusion found to be relatively ineffectual in maintaining the initial distribution, especially for smaller plate separations. In the far more unstable stratification in (b), in which a Taylor-type instability related to the acceleration of a density interface toward the heavier medium arises, diffusional processes playa perturbational role relative to inertial processes, except at small disturbance wavelengths. An energy approach to describing the finite-amplitude overturning of case (b) is suggested as a means of estimating the width of protuberances which grow to large size.