A novel Schiff base Nʹ-[1-(3-aminophenyl)ethylidine]isonicotinohydrazide was prepared and its complexation behavior towards some selected lanthanides had been studied employing pH-metric and calorimetric titration and spectral techniques. pH-metric studies were carried out for the trivalent La, Pr, Nd, Sm, Eu, and Gd complexes in 30% aqueous-dioxane medium at constant ionic strength of 0.05 mol/L NaClO4 and at different temperatures of 293, 303 and 313 K. The proton-ligand formation constants of the ligand indicated the presence of only one dissociable proton while the metal-ligand formation constants were compatible with the formation of 1:1 Ln(III) complexes. The stability of the complexes followed the order: La3+
Gd3+, showing a break at gadolinium. The thermodynamic parameters, ΔG, ΔH and ΔS associated with protonation and complexation reactions were negative which suggested that all reactions were exothermic and enthalpy-driven. Isothermal calorimetric studies of Gd3+-aeINH systems at 303 K also showed exothermic nature of the complexation reaction and formation of 1:1 complex in agreement with the pH-metric data. Formation of 1:1 complexes was confirmed by the characterization of Nd(III) complex. A seven coordinated geometry was assigned for the complex based on its elemental and spectral data.