In an earlier paper (Ghose A. K.; Crippen, G. M. J. Comput. Chem. 1986, 7, 565) the need of atomic physicochemical properties for three-dimensional-structure-directed quantitative structure-activity relationships was demonstrated, and it was shown how atomic parameters can be developed to successfully evaluate the molecular water-1-octanol partition coefficient, which is a measure of hydrophobicity. In the present work the atomic values of molar refractivity are reported. Carbon, hydrogen, oxygen, nitrogen, sulfur, and halogens are divided into 110 atom types of which 93 atomic values are evaluated from 504 molecules by using a constrained least-squares technique. These values gave a standard deviation of 1.269 and a correlation coefficient of 0.994. The parameters were used to predict the molar refractivities of 78 compounds. The predicted values have a standard deviation of 1.614 and a correlation coefficient of 0.994. The degree of closeness of the linear relationship between the atomic water-1-octanol partition coefficients and molar refractivities has been checked by the correlation coefficient of 89 atom types used for both the properties. The correlation coefficient has been found to be 0.322. The low value suggests that both parameters can be used to model the intermolecular interaction. The origin of these physicochemical properties and the types of interaction that can be modeled by these properties have been critically analyzed.