The first complete microequilibrium analysis of a tetrabasic acid is presented. A total of 18 protonation microconstants of glutathione disulfide (GSSG) were determined, 16 of them quantitate the overlapping equilibria in acidic medium, where 4 carboxylate sites protonate. The related, pH-dependent concentrations of 16 coexisting microspecies were also obtained, including the 10 different ones and their 6 identical twins of symmetry origin. The GSSG acid-base chemistry in basic medium is also characterized. It is shown for tetra- and further multibasic acids that the number of traditional microconstants mandatorily exceeds that of the pieces of independent information available by any state-of-the-art methodology, making the complete microequilibrium resolution not only highly complex but theoretically impossible on the basis of hitherto reported principles. The GSSG microspeciation was made feasible here by introducing cumulative microconstants, a new equilibrium parameter, and the invariance of the interactivity coefficient, a plausible simplifying principle, on the experimental basis of H-1 NMR-pH titrations in H2O/D2O 9/1 media. The methods and results were verified by corresponding data of GSSG-(glycyl)-diethyl-ester, a model compound of reduced microequilibrium complexity. The obtained log k(U) = 2.44 and log k(Y) = 3.59 "core" microconstants of the respective glutamyl and glycyl carboxylates of GSSG are close to,those of the sulfhydryl (GSH) form of glutathione. The GSSG intermoiety interactions were found to be weak, apparently of Coulombic nature. The pH-dependent distribution of the 18 microspecies is depicted.