The protonation equilibria of penicillamine and its complex formation with the TI(I) ion were studied over a wide range of pH (I to II), using a combination of spectrophotometric and potentiometric methods at constant temperature, 25 degrees C, different methanol-water mixtures [(0 to 45) % v/v], and constant ionic strength (0 1 mol.dm(-3) sodium perchlorate) Least squares regression calculations are consistent with the formation of TIH(2)L(+), TIHL and TIL(-) species. where L(2-) represents the fully dissociated ligand The protonation of penicillamine and the formation constants of the formed complexes in different media were analyzed in terms of Kamlet. Abboud, and Taft (KAT) parameters. A single-parameter correlation of the formation constants vet sus a (hydrogen-bond donor acidity), beta (hydrogen-bond acceptor basicity), and pi* (dipolarity/polarizability) are relatively poor in all solutions, but multiparameter correlations represent significant improvements with regard to the single-parameter model Linear correlation is observed when the experimental log beta(xyz) values are plotted versus calculated ones, while all of the KAT parameters arc considered. Finally, the results are discussed in terms of the effect of solvent on protonation and complexation