It nas long been Known that the molar refractions or solutions, R(v) = {(n(2) - 1)/(n(2) + 2}V-v(m), can be estimated from those of the separated components using additivity relations, but deviations from such rules have been large enough to prevent practical application (such as the calculation of excess volumes from excess refractive index data, or vice versa). The present paper develops an empirical correction to additivity resulting from asymmetry in the concentration dependence of the dispersion energy, thus permitting practical application of additivity rules for R(v). With this new approach, it becomes possible to use conveniently obtained continuous-dilution differential refractometry data (c.d.d.r.) to yield both excess molar refractions and excess volumes over the entire concentration range.