Potentiometric equilibrium measurements were made for some metal ions (M(II) = Co(II), Ni(II), Cu(II), Zn(II), Cd(II), Ca(II), Sr(II), and Ba(II)) with guanine (A) in a 1:1 (M(II):A) ratio and with cytosine, cytidine, 5-bromocytosine, 5-azacytosine, and 5-fluorocytosine as primary ligands (L) and guanine as secondary ligand in a 1:1:1 (M(II):L:A) ratio at (25.0, 35.0, and 45.0) degreesC and I = 0.1 mol.dm(-3) NaNO3 in aqueous solution. The experimental pH-titration data were analyzed by using a BEST computer program in order to evaluate the formation constants of various intermediate species and their relative distribution. The experimental conditions were selected in such a way that the self-association of the nucleobases and their complexes due to stacking interaction was negligibly small, so that only the neutral monomeric and hydroxo ternary complexes were studied. The enthalpy (Delta(f)Hdegrees) and entropy (Delta(f)Sdegrees) changes for the formation of binary and ternary complexes were calculated from temperature coefficient data. The deltaDelta(f)Sdegrees values are positive for all the metal ligand systems. The negative deltaDelta(f)Hdegrees values indicate the extra stabilization of most of the ternary complexes by the exothermic enthalpy change (deltaDelta(f)Sdegrees = Delta(T)Sdegrees - Delta(B)Sdegrees and deltaDelta(f)Hdegrees = Delta(T)Hdegrees - Delta(B)Sdegrees where Delta(T)Sdegrees, Delta(T)Hdegrees and Delta(B)Sdegrees, Delta(B)Hdegrees are the entropy and enthalpy values associated with the ternary and binary complexes, respectively). On the basis of IR data for metal complexes with the 5FC-G mispair, it has been proposed that the guanine is bonded to metal ions through N1/C6=O and N7, whereas cytosine and its derivatives are bonded through N3 atoms in ternary complexes.