Thermodynamic and transport properties of hydrocarbon systems are determined, in part, by the van der Waals attractions between molecules. These van der Waals attractions (or London dispersion interactions) are related to the electronic polarizability and molar refractivity of the molecules. Molar refractivity is related to refractive index and molar volume through the Lorentz-Lorenz equation. In this work we present a method that relates the refractive index of a hydrocarbon substance with its mass density. This is based on the observation that the molar refractivity is approximately proportional to the molecular weight of a hydrocarbon molecule. The proportionality constant is approximately equal to one-third for hydrocarbons and crude oil systems. Although both the refractive index and the mass density are functions of temperature, it has been found that the proportionality constant of this model is nearly invariable over a wide range of pressures and temperatures. Therefore, given the temperature dependence of one of the variables, it is possible, by applying the "One-Third" rule, to estimate the temperature dependence of the other variable. This correlation has been validated with over 200 crude oil samples, in a wide range of densities (10-55 degrees API) and temperatures (10-70 degrees C), and the results obtained are remarkable. The applications of the One-Third rule in the calculation of properties such as solubility parameter, viscosity, thermal conductivity, diffusivity and heat capacity of hydrocarbons and crude oil systems, as a function of the mass density, are presented and discussed. This approach also provides an alternative to calculate the refractive index based on densities obtained from an equation of state. The method presented in this work is promising in offering a new, simple and reliable method for estimating hydrocarbon and crude oil properties as a function of their mass density. (C) 2009 Elsevier B.V. All rights reserved.