A four-parameter corresponding states expression has been used, together with two new parameters proposed in this work. The corresponding states expression is general and relaxes the frequent restriction that one or more of the reference fluids should have parameter values equal to zero. The new third parameter, a shape-size parameter, is based on fundamental molecular properties such as bond lengths and bond angles. The fourth parameter is a polarity parameter that takes both dipole and quadrupole moments into account. Since the parameter estimation is based only on a small number of substances, the results should be regarded as preliminary. Calculations of vapor pressure show that the corresponding states expression proposed in this work, together with the two new parameters, give deviations of the same order as a three-parameter expression with Pitzer's acentric factor. The effects due to geometry and polarity, which are both included in the acentric factor, are thus separated into two parts.