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Published in Petroleum Transactions, AIME, Volume 219, 1960, pages 288–292. Abstract One of the major difficulties in predicting the performance of oil reservoirs from their early pressure history lies in the uncertainty of estimating the volume of the liquid hydrocarbons contained in them. As a first step in filling this need, an equation was developed to determine the molal liquid volume of pure hydrocarbons over a wide range of temperature and pressure. The second step consisted of adapting the equation to apply to mixtures, with the heavy hydrocarbons expressed as C7+. The equations are similar in form to van der Waals' equation, but the constants a and b are considered as functions of temperature. In addition to the gas constant R, there are four constants characteristic of each hydrocarbon. When compared with experimental values found in the literature, the average absolute deviation in the calculated molal volumes is found to be a maximum of 0.33 per cent for any of the pure liquid hydrocarbons studied. This maximum deviation was that found when comparing the calculated and observed values over a temperature range of 86° to 482°F and a pressure range from the bubble-point to 10,000 psia. The equations expressing the correlation for mixtures were developed from 647 experimental measurements of volume on 47 bottom-hole samples covering a temperature range of 72° to 250°F and a pressure range from bubble-point to 5,000 psig. The average absolute deviation was found to be 1.6 per cent with the maximum for any measurement of 4.9 per cent. Introduction Accurate information of the pressure-volume-temperature behavior of hydrocarbon liquids is of considerable importance in the field of both applied and theoretical science and, especially, in the solution of petroleum reservoir engineering problems. These PVT relationships can be expressed graphically, in tabular form or as equations of state.