A thermodynamic approach based on the regular solution concept is applied to the description of miscibility gap boundaries in the alkali-borate systems. It is suggested that in each system the structural units which control the entropy of mixing are the stoichiometric compounds at the apparent limit of the alkali-rich edge of the gap, and a complex boron trioxide structure. (The former is inferred by the shape of the gap, and the latter is chosen to fit the regular mixing equation.) The same boron trioxide complex is used for all the systems analyzed. The physical implications of this analysis are discussed.