Published in Petroleum Transactions, AIME, Volume 210, 1957, pages 40–47. Abstract This paper discusses a new oil recovery process called the "miscible slug process." This process involves the injection of propane or LPG into the reservoir prior to gas injection. The operating conditions are maintained so that the slug is miscible with both the reservoir oil and the injected gas phase. Thus, a miscible phase displacement is achieved, and high recoveries are obtained from the area of the reservoir contacted. The most striking discovery of the laboratory study which included displacements from cores up to 125 ft in length was that relatively small slugs of LPQ are effective over reservoir distances in the miscible displacement of oil. This snakes possible the commercial-application of the process. The important factors controlling the size of the slug are:reservoir length,reservoir fluid composition, andreservoir pressure at the displacement front. Of lesser importance are the effects of injection rate and porous medium type. Questions which have not been completely answered deal with the effects of gravity and reservoir inhomogeneities on the process. Introduction One of the major problems facing the oil industry today is recovery of the large fraction of the discovered oil which will remain unrecovered unless some new processes are found. Water-flooding has been one successful method used to increase recovery over natural depletion. But even successful water floods may leave 25–40 per cent of the pore space filled with residual oil. Recently a process which involves the use of thermal energy has been reported which may prove useful in increasing recovery from shallow reservoirs containing highly viscous crudes. This paper deals with a new oil recovery process involving use of miscible phases to displace the oil from the reservoir. Miscible phase displacement of oil has been an intriguing idea because the elimination of capillary effects in the reservoir leads to 100 per cent recovery in the areas contacted by the miscible displacing phase. The big deterrent in the past to the practice of miscible phase displacement has been the high cost involved in using large volumes of solvent in the process. The use of high pressure gas to achieve a miscible phase displacement of reservoir oil has been reported as one economically feasible method. However, this process calls for the maintenance of pressures above 3,000 psi at the displacement front.