THEprediction of thermodynamic properties of mixtures of light hydrocarbon vapors has long been a troublesome problem in the petroleum and associated industries. The ability to calculate the enthalpy of various grades of gasoline, for instance, would be of great value in petroleum processing. With the information available at present, however, such a calculation is not possible. If reasonably precise calculations of this type are to be made possible, fundamental data in this area are needed.