A new way to define the two parameters a, b of the Redlich- Kwong equation of state without using the critical constants is introduced First, the "shifted" covolume (b-c) of the Peneloux modification of the equation of state is estimated by group contributions (for a wide range of compounds); from it and one vapor pressure-liquid density pair, the covolume b is then determined. The attractive parameter a is derived at any temperature, from the covolume and the compound vapor pressure. The proposed method makes it possible to treat substances whose critical constants are not known (heavy or thermally unstable compounds) by using one density value and the vapor pressure curve. Examples are given of vapor-liquid equilibrium calculations.