In this paper we show application of the refractive index in the estimation of various thermodynamic and physical properties of pure hydrocarbons and undefined petroleum mixtures. In particular, we demonstrate how one can use the refractive index to estimate equation of state parameters, critical constants, the composition of petroleum mixtures, and temperature-dependent properties such as heat capacity, PVT, and transport properties of hydrocarbon fluids. The refractive index is an easily measurable property; however, when it is not available, methods of estimation of this property through other available parameters are also presented.