We propose a method for deriving Stockmayer potential parameters for polar fluids, based on phase-coexistence data deduced from computer simulation results for Stockmayer fluids. The Stockmayer parameters for a particular fluid are derived by relating the critical temperature and one liquid density point of the real fluid to those of a corresponding Stockmayer fluid. Significant improvement in the prediction of the vapour-liquid coexistence curve is achieved over other parameter sets found in the literature. The proposed method yields parameter sets that have more physical significance than those of the second-virial-coefficient method, and is more discriminative than the gas-viscosity method. In addition, the method requires little experimental input (combined in a parameter called K) and is easy to apply. It is recognized that K is the key parameter in defining the reduced dipole moment. In thermodynamic modelling work, may be taken as the definition of a ’polarity factor’, analogous to the acentric factor.