AbstractThe physical processes are discussed by which a fluid is displaced from a porous medium during steady state viscous flow by another fluid of the same density and viscosity under conditions of complete miscibility of the two fluids. The displacement occurs on a microscopic scale as a result of combined convective and diffusional mixing. The length of the zone of mixing which comprises the displacement front is predicted to be dependent upon the rate of flow, the diffusion coefficient for the two‐fluid system, the characteristics of the pore geometry, and the distance the front has traversed at the time of its observation.Experimental data are presented for the displacement of benzene by ethyln‐butyrate at several rates of flow from packed sand columns. These data show that the length of the frontal mixing zone after a prescribed distance of flow is greater at the higher rates of flow. The postulated dependence of the length of the front upon the diffusion coefficient and the pore geometry has not yet been investiga