Abstract A description is given of the theoretical background to the optical basicity concept, which is based on orbital expansion effects reflecting the nature of the chemical bonding between a Lewis acid-base pair. Optical basicity is particularly useful for oxidic network systems, such as silicate, phosphate, or borate in crystalline, vitreous, and liquid (molten) form. The experimental measurement of optical basicity, from the ultraviolet spectra of probe ions such as Tl + and Pb 2+, is described, and it is shown how optical basicity values can be assigned to individual oxides such as Na 2O, CaO, or SiO 2. These make it possible to calculate a “theoretical” optical basicity value, Λ th , from chemical constitution, thereby allowing correlations to be made with various properties of, for example, molten silicates as a function of composition. The correlations have important applications for redox behaviour, e.g., in glass making and in the performance of metallurgical slags, and should find application in geochemistry, for example, to Fe 2+ Fe 3+ and other redox ratios in magmas. Furthermore, in the chemical reactions between metal silicate, borates, etc., the Λ th values indicate how minimisation or elimination of optical basicity differences between the reactants is a major driving force.