Inspired by the work of John Duffy on optical basicity of oxyfluoride glasses, we apply here the conceptof optical basicity to oxynitride systems. While in the original work of Duffy and Ingram the basicity of amedium could be probed by s2 ions like Pb2+, the low energy intrinsic absorption edge of nitridecontainingsystems does not allow the use of such probe ions. This study uses therefore experimentaldata on refractive index and density of alkaline earth and rare earth containing silicate oxynitride glasses,prepared by the authors or taken from the literature. In addition, literature reports on experimental orcalculated refractive index, density and polarizability data are used to compare pure nitride systems, e.g.bulk or thin film materials that are either crystalline or glassy. We compare simple and complex nitridesystems with their oxygen counterparts, by calculating their optical basicity using the chemicalcomposition as well as the established relationship between optical basicity, L, and electronicpolarizability in oxide systems. Our results on oxynitride systems are in good agreement with Duffy’sprevious work on oxyfluoride glasses and indicate that the optical basicity varies for the isoelectronicanions in nitrides, oxides and fluorides (N3:O2:F) of a cation Mm+ as follows: L(MFm) = 1/2L(M2Om) =1/3L(M3Nm). Using this relation for CaO, for which the optical basicity was set as unity by Duffy andIngram, one has L(CaF2) = 0.50, L(CaO) = 1.00 and L(Ca3N2) = 1.50. The optical basicity of complexnitrides can therefore be calculated by the same method established for oxides using the equivalentfractions and the basicity of the constituent nitrides. The relationship between nitride polarizability aNand basicity L(nitride) was found to be linear, with L(nitride) = 0.39aN 0.14 where aN is given in Å3.