The interaction of guanosine 5 -monophosphate, GMP, with the thallium(I) ion was studied by UV-vis and potentiometric titration methods and 31 P NMR spectroscopy. Both NMR spectra and UV-vis titration data have shown that GMP coordinates via guanine to the thallium(I) ion in the pH range 1.5-10. Our study of the system Tl(I) + GMP was performed in water-methanol mixtures with different volume ratios of methanol. The complexation equilibrium in the pH range of study led to the following mononuclear species: TlH 2 (GMP) + , TlH(GMP) and Tl(GMP) − , where (GMP) 2− represents the fully dissociated ligand. The formation constants of the species were calculated in the various media at constant temperature (25 • C) and constant ionic strength of sodium perchlorate (0.1 mol dm −3 ) using a suitable computer program. The formation constants were analyzed in terms of Kamlet and Taft's parameters. A single-parameter correlation of the formation constants, β 121 , β 111 and β 101 vs α (hydrogen-bond donor acidity), β (hydrogen-bond acceptor basicity) and for π * (dipolarity/polarizability) are relatively poor in all solutions, but multi-parameter correlations represent significant improvements with regard to the single-parameter model. In this work, we have also used the normalized polarity parameter, E T N , alone and in combination with some of the Kamlet-Taft parameters to find a better correlation of the formation constants in different methanol-water mixtures.