Data for the density and refractive index of mixtures of acetone and water are presented. Deviations from ideality for such data are clearly shown if data are correlated with ideal volume per cent composition. If there is no deviation from ideality, density and refractive index may be exactly calculated from the ideal volume per cent composition. With deviation from ideality, a plot of refractive index vs. density in which the actual points are connected with the ideal points is very revealing. The slope of these tie lines may be related to the slopes corresponding to various specific refraction equations In the case of the acetone-water blends, the simple Gladstone and Dale relation accurately represents the change of refractive index in relation to the change of density owing to the contraction on mixing. The possible effect of hydrogen bonding on the relation between refractive index and density for binary aqueous mixtures is pointed out, The presence of hydrogen bonding, or absence of it, may determine which specific refraction function best represents the data for a particular aqueous binary mixture.THE study of data for binary mixtures of liquids, and of simple solutions of solids in liquids, is important for developing a better understanding of the liquid state (5, 6, 20, 2 2 , 2 3 ) . T h e properties of binary mixtures of liquids, particularly aqueous solutions, cannot be predicted with certainty from the properties of the pure materials but must be quantitatively evaluated from experimental d a t a (5, 6. 20, 22, 23). This paper contributes d a t a on binary mixtures of acetone and water and discusses briefly the application of refraction functions to the generalization of such data. D a t a on binary mixtures of acetone and water were obtained some time ago for analytical use. These data have been checked recently in a second laboratory. These two sets of data are presented in Tables I and 11. T h e composition d a t a are recorded both a s ideal volume "C and weight "c. Deceased.Mole per cent composition is used in many discussioris of binary mixtures (19. 21. 22, 28), b u t consideration from the volume c70 point of view still has some advantages. T h e space separating molecules is, of course, closely related to volume clc composition, Table 111 compares three sets of data a t 50.26 ideal volume 5 water. T h e agreement among the three sets of data is satisfactory. Table IV compares the experimental density and refractive index data for the acetone and water used in this work with literature data. A s the agreement is good, one may conclude that the materials used were of adequate purity. Ideal volume % is the volume cc calculated from the weight 5 assuming no volume change on mixing. By definition, density must plot linearly against ideal volume CC if there is no volume change on mixing (13, 19. 23. 2 5~, therefore the deviation from linearity when density 1s plotted against ideal volume CC is a direct measure of deviation from 330 JOURNAL OF CHEMICAL AND ENGINEERING DATA