Compressors in oil and gas applications operate at high pressures and moderate temperatures, compressing gas mixtures with a large number of constituents, including carbon dioxide, nitrogen, methane, ethane and other heavier hydrocarbons, as well as, water. Equations of State are used to determine the performance of compressors and pipeline hydraulics. These are semi-empirical models that allow the calculation of thermodynamic and physical properties, such as density, enthalpy, and entropy, of gas mixtures for known pressures and temperatures. While a large body of work is available comparing the results from different equation of state models, very little data is available to verify the results from the various equation of state calculations for the range of pressures, temperatures and gas compositions relevant to many oil & gas operations. This is especially true for dense phase (supercritical) natural gas and gases containing carbon dioxide, hydrogen sulfide, or heavy hydrocarbons. Thus, the users of equations of state (operators and manufacturers) often have no precise data to determine which equation of state gives the most accurate results when predicting compressor performance. A novel method and test apparatus was developed to measure enthalpy rise directly using a calibrated near isentropic compression process. In this apparatus a test gas is compressed using a fast-acting piston inside an adiabatic autoclave. Test results are then corrected using calibration efficiencies from a known reference gas compression process at a similar Reynolds number. By using the near isentropic compression cylinder apparatus and the calibration factors (which are determined for a set of reference Reynolds number compression processes) the enthalpy rise of any gas mixture can be accurately determined over a wide pressure and temperature range. This paper describes the test apparatus, the measurement methodology, and the test results for a complex hydrocarbon gas composition at elevated temperatures and pressures. An uncertainty analysis of the new measurement method is presented and results are compared to several equations of state.