The complexation of N'-[1-(2,4-dihydroxyphenyl)ethylidene]isonicotinohydrazide (DER) with trivalent Pr, Nd, Gd, Tb, and Ho ions were studied employing potentiometric and spectroscopic techniques. The potentiometric studies carried out at different temperatures, (293.15, 303.15, and 313.15) K, in aqueous dioxane (30 %) medium at a constant ionic strength of I = 0.1 mol . dm(-3) KNO3 indicate the presence of two ionizable protons in the ligand. The formation of Ln(III) complexes having 1:1 and 1:2 metal ligand stoichiometry has been inferred from the metal-ligand formation curves and the elemental analysis data. The trend in the stability of the Ln(III) complexes follows the order: Pr < Nd < Gd > Tb < Ho. Thermodynamic parameters (Delta G, Delta H, and Delta S) of both protonation and complexation reactions are negative for all of the systems suggesting that the reactions are spontaneous, exothermic, and enthalpy-driven. A photoluminescence study reveals that the complexes are not luminescent due to the quenching effect of the ligand or aqueous or OH ions. The Tb(III) complex of the ligand isolated from ethanolic medium is paramagnetic, behaves as a 2:1 electrolyte, and is of 1:2 stoichiometry.