Potentiometric equilibrium measurements have been pet-formed at (25.0 +/- 0.1) degreesC and ionic strength I = 0.1 mol dm(-3) (KNO3) for the interaction of guanosine 5'-monophosphate, inosine 5'-monophosphate, and cytidine 5'-monophosphate and Cu(II), Ni(II), Co(II), Mn(II), Zn(II), Pd(II), Ca(II), and Mg(II) with the biologically important secondary ligand zwitterionic buffers 3-[N-morpholinol]-2-hydroxypropanesulfonic acid, 3-[N-tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid, and N-2-acetamido-2-aminoethanesulfonic acid in a 1:1:1 ratio. The experimental conditions were selected such that self-association of the nucleotides and their complexes was negligibly small; that is, the monomeric complexes were studied. The formation of various 1:1:1 mixed ligand complexes was inferred from the potentiometric titration curves. Initial estimates of the formation constants of the resulting species and the acid dissociation constants of guanosine 5'-monophosphate, inosine 5'-monophosphate, cytidine 5'-monophosphate, and the secondary ligands 3-[N-morpholinol] -2-hydroxypropanesulfonic acid, 3-[N-tris(hydroxymethyl)methylamino]-2-hydroxypropanesulfonic acid, and N-2-acetamido-2-aminoethane sulfonic acid have been refined with the SUPERQUAD computer program.