A high-precision light-scattering photometer has been built to measure the relative intensity of the light scattered by pure solvents with an accuracy of 0.1 %. Taking the necessary steps to show that the geometrical corrections can be calculated to that precision, it has been possible to study systems of which the intensity of scattering is low and where the scattering arises mainly from the density fluctuations. Because of this, one cannot neglect the variation with concentration of the density fluctuations scattering, as is customary for light-scattering molecular-weight determination, and it is no longer possible to assume that the density and the concentration fluctuations are additive. By taking these two effects into account, the light scattering data from binary liquid mixtures are used to calculate the Rayleigh ratio of the pure solvent for systems where the molecular weight of the solute is known. Results are given for 5461 A, at 25"C, for benzene (17.2~ 10-6), carbon tetrachloride (6.0 x 10-6) and diethylether (5-25 x 10-6) and for benzene at 4358 8, (49 x 10

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