N-(Pyridin-2-ylmethylene)isonicotinohydrazide, pmINH, has been synthesized and characterized on the basis of elemental and spectral data. The protonation constants of pmINH and the stability constants of the base with trivalent La, Pr, Nd, Sm, Eu, and Gd at constant ionic strength (I, mol center dot dm(-3) = 0.05 M NaClO(4)) and at different temperatures, T= (293, 303, and 313) K, were determined potentiometrically in water-dioxane (30%) medium. The ligand forms only 1:1 complexes with the lanthanides. Both protonation and complexation reactions are found to be exothermic in nature. The trend in the formation constants follows the order: La(3+) < Pr(3+) < Nd(3+) < Gd(3+) < Sm(3+) < Eu(3+) and shows a break at gadolinium. The thermodynamic parameters associated with the complexation reactions were also calculated. The values of Delta G, Delta H, and Delta S are negative for all the systems and suggest that all reactions are enthalpy driven. The thermodynamic parameters of the complexation are correlated with the reciprocal ionic radii of the metal ions. Raw and normalized isothermal calorimetric data for the titrations of Gd(3+)-pmINH solutions at 303 K show that the reaction was exothermic and metal-ligand stoichiometry to be 1:1.