Potentiometric equilibrium measurements have been performed at (25.0 +/- 0.1) degrees C and ionic strength I = 0.1 mol.dm(-3) KNO3 for the interaction of glycylglycine, glycylhistidine, glycylglycylglycine, glycylhistidylglycine, and Al(III), Ga(III), In(III), and Tl(I) with the biologically important secondary ligands bicine [N,N-bis(2-hydroxyethyl)glycine], tricine [N,N,N-tris(hydroxymethyl)methylglycine], PIPES [piperazine-1,4-bis(2-ethanesulfonic acid)], HEPES (N-[2-hydroxyethyl]piperazine-N-[2-ethanesulfonic acid]), and EPPS 4-(2-hydroxyethyl)piperazine 1-propanesulfonic acid in a 1:1:1 ratio. Ternary complexes of the type Al(III)-glycylglycine-guanosine, Al(III)-glycylglycine-guanosine 5'-monophosphate (GMP), Al(III)-glycylglycylglycine-guanosine, Al(III)-glycylglycylglycine-GMP, Al(III)-glycylhistidine-guanosine, Al(III)-glycylhistidine-GMP, Al(III)-glycylhistidylglycine-guanosine, Al(III)-glycylhistidylglycine-GMP in a 1:1:1 ratio have been investigated. The experimental conditions were selected such that self-association of the nucleotide and their complexes was negligibly small; that is, the monomeric normal and protonated complexes were studied. The formation of various mixed ligand complexes was inferred from the potentiometric titration curves. Initial estimates of the formation constants of the resulting species and the protonation constants of the different peptides and zwitterionic buffers used have been refined with the SUPERQUAD computer program. Confirmation of the formation of ternary complexes of the type M-P-Z in solution has been carried out using differential pulse polarography, square wave voltammetry, cyclic voltammetry, and UV spectroscopic measurements.