The formation of complex anions in molten halide systems is treated semi-quantitatively by a discrete complex anion model. The treatment is a more general application and extension of an approach reported previously by the author. By assuming the existence of discrete MY42− tetrahalo complex anions, the model gives good agreement to experimental enthalpies and entropies of mixing and electrical conductivities in a large number of different halide systems of the general formula MY2–AY, where M is a divalent metal, and A is an alkali. Very few adjustable parameters are required. The parameters provide semi-quantitative basicity (or acidity) scales by which molten salts and slags may be classified. The relationships of these basicity scales to ionic radii, ligand field stabilization energies, etc. are shown. General thermodynamic and mathematical aspects of the problem of complexing are discussed, particularly as regards the proper method of calculating the degree of dissociation of the complexes.