Working on a mesoscopic scale, a new cubic expression for the effective permittivity of mixtures as a function of the mean permittivity fluctuation is established. Using previous methods for calculating this mean value, the effective permitivity of a mixture can thereby be written as a function of the fractional volumes of its components and their pure permittivities. A comparison with experimental results for (tetraglyme + n-nonane) near the consolute point with some other known expressions for mixtures are presented.