An investigation made into the variation of oxide(-II) electronic polarizability, alpha(oxide(-II)), for glasses covering a wide range of composition shows that there is an almost linear relationship between alpha(oxide(-II)) and glass basicity (expressed as optical basicity, A). However, systematic deviations pinpoint the role of coordination number on the polarizing power of cations such as Al3+, Ge4+, and Mg2+. Data for glasses containing these cations indicate that the oxide(-II) atoms of the glass undergo the same degree of polarization regardless of whether these cations exercise a coordination number of four or six. The results of previous studies on cationic charge trends for Fe3+ in glass are now explicable on this basis. Furthermore, it is shown that when the effect of coordination number is taken into account in the calculation of A for crystalline metal oxides (such as Al2O3 or GeO2) and also metal silicates and alummosilicates, there is good agreement with the alpha(oxide(-II))/Lambda trend already established for glasses. The optical basicity model is currently applied to technical problems concerned with metallurgical slag performance and also glass melts, and the present findings are expected to have important implications for these materials.