Artificially consolidated sand models, representing one-quarter of a five-spot, have been developed and used to study factors affecting miscible displacement. Sweep efficiency at breakthrough, size of the mixing zone between two miscible liquids and per cent are contacted by drive after breakthrough were determined for the high mobility ratios encountered in actual reservoirs. Quantitative relationships between the degree of viscous fingering and mobility ratio were obtained by measuring the length of the fluid interface. Scaled miscible-slug experiments, supported by field evidence reported in the literature, have shown that when the slug is followed by dry gas the process is less efficient than expected. In addition to low areal sweep efficiencies encountered for high mobility ratio displacements, the effectiveness of a miscible slug is greatly reduced. This is caused by an accelerated growth of the mixing zone between the driving and the displacing fluids. Introduction Considerable interest in miscible displacement as a secondary recovery process has been shown by the oil industry in recent years. In addition to extensive laboratory investigations, numerous field operations have been initiated so that 39 miscible displacement projects had been reported in the United States by early 1959. By far the most popular approach in these projects is the use of an LPG slug followed by dry gas.