A simple, molecular size-based model is presented that allows unified description of solubilities, partition coefficients, vapor pressures, enthalpies of vaporization, and boiling points for a variety of simple organic liquids where no specific interactions are present. A free energy expression obtained from specific, molecular-level assumptions is used. This makes it unnecessary to rely on standard state and activity concepts, and it also leads to relations that cannot be obtained from purely thermodynamic arguments. Molecules in the liquid phase are considered as moving in a fraction of the volume not excluded by their own size and in an average attractive potential of the surrounding molecules that can be described by molecular volume through a simple, linear relationship. These assumptions allow the development of a model that gives unified and reasonably good description of organic liquids that have no hydrogen bonding or strongly polar substituents. In practically all of the individual correlations presented, molecular size alone as measured by computed van der Waals molecular volume, accounts for more than 90% of the variance in these properties. In addition, interaction constants derived from enthalpies of vaporization can describe not only boiling points but partition and solubility properties as well. A previously described, fully computerized method that estimates octanol-water partition properties for a large variety of organic solutes (Bodor, N.; Buchwald, P. J. Phys. Chem. B 1997, 101, 3404) can also be integrated within this approach.