This work gives an overview of the changes which have helped improve the basic Peng-Robinson equation of state (EOS). Four pure compound properties are considered : vapor pressure, saturated liquid volumes, saturated vapor volumes, and enthalpies of vaporization. A comparative study of the results obtained with the basic forms of the Peng-Robinson, the van der Waals, the Redlich-Kwong, and the MCSV equations of state clearly shows the advantages of the Peng-Robinson form.