To improve the SRK equation of state for the calculation of vapor-liquid equilibria of petroleum fluids, the parameters of the Mathias-Copeman alpha-function were refitted to a large number of experimental vapor-pressure data (15,311 data points) stored in the Dortmund Data Bank with a particular view to improving the accuracy of the vapor-pressure correlation in the low pressure range. Using the new parameters, vapor-liquid equilibria were calculated using two different mixing rules: the group contribution mixing rule for the mixture parameters a of the SRK EOS; the conventional mixing rules for the mixture parameters a and b of the SRK EOS together with a characterization method developed on the basis of existing empirical correlations for the properties of plus fractions of undefined mixtures. The results show that the accuracy for crude oil and natural gas mixtures can be improved by refitting the SRK EOS parameters.