The design and operation of an interferometric continuous-dilution differential refractometer (c.d.d.r.) is described. For typical solutions, and even for solutions of isotopomers, the refractive index differences between solution and reference (usually one or the other pure component), are large enough to give interferograms amounting to many fringes. Deconvolution yields profiles of refractive index against concentration which can be interpreted straightforwardly in terms of the refractive indices and molar volumes of the pure components and the excess volume of the solutions. The c.d.d.r. method is illustrated with new refractive index data for (water + deuterium oxide), (acetone + d(6)-acetone), and (water + acetone) at T = 298.15 K and lambda = (470, 520, 559 and 650) nm.