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A four-parameter corresponding states correlation has been developed for the prediction of thermodynamic properties of nonpolar and polar fluids. The property of a fluid is given as a Taylor's series expansion about the simple fluid property at the same reduced conditions in terms of geometric and polar deviations. Three reference fluids are used to evaluate the terms of the series. The first, a simple fluid, and the second, an acentric fluid, are adopted from the Lee-Kesler three-parameter correlation. The third, a polar fluid, is water, represented by the Keenan and Keyes equation of state. The four parameters required are the critical temperature and pressure, and two newly developed parameters which account for geometry and polarity, respectively. The geometric parameter is obtained from the radius of gyration, and the polarity parameter is derived from one liquid density point, or alternatively from the heat of vaporization (for derivative properties). The method, in the form of an easily used computer program, is used to obtain estimates of compressibility factors, enthalpy and entropy departure functions, and fugacity coefficients. The temperature and pressure of the fluid are specified, upon which an estimate of the desired property is calculated. The phase of the fluid may also be specified, but if unknown it is determined from a newly developed vapor-pressure correlation which is an extension to polar fluids of the Lee-Kesler nonpolar-fluid correlation. Extensive tables are also given for hand calculations. The method has been tested on both nonpolar and polar fluids and yields results equivalent to those obtained by the Lee-Kesler three-parameter method for nonpolar fluids and better results than existing methods for polar fluids. Average errors for the compressibility factor of polar fluids for all points tested are 1.9% for the vapor phase and 1.6% for the liquid phase. Average errors for enthalpy departure functions are 250 J/mol for the vapor phase and 422 J/mol for the liquid phase. Preliminary mixture results indicate that the method will be effective for mixture property predictions as well.