The frequency-dependent polarizabilities of a number of halogen-substituted benzene species are computed using ab-initio methods. The polarizability tensors are analyzed for possible additive properties. Simple group or atom additivity trivially fails (the overall polarizability is not a sum of isotropic atom or group polarizabilities). If, however, the separate polarizability tensor elements are studied, they are additive, to quite substantial accuracy, for benzene with F, Cl, Br, and I substituents. This tensorial additive property has important implications for design of controlled refractive index materials, and we present calculated refractive indices utilizing ab initio calculated polarizability tensors.