Spectrophotometric and potentiometric equilibrium measurements of hydrogen ion concentration have been employed for the determination of acidity constants of (E)-N'-(2-hydroxy-3-methoxybenzylidene)benzohydrazide (H(2)L) in 0.9 volume fraction of methanol solution at (25.0 +/- 0.1) degrees C and an ionic strength of 0.10 M tetraethylammonium bromide (TEAB). In the spectrophotometric method, only the acidity constant of the hydroxy group that is directly attached to the aromatic ring (phenolic group) was determined, whereas, by the potentiometric method, the acidity constants of both hydroxy groups could be determined. The pK(a1), values obtained for the first acidity constant by both techniques, potentiometry and spectrophotometry, are 9.03(+/- 0.05) and 9.07(+/- 0.04), respectively, which are in good agreement. The pK(a2) value obtained for the second acidity constant by potentiometry is 10.92(+/- 0.07). The complexation reaction of the ligand with Fe(III) ions in 0.9 volume fraction of methanol solution was also investigated by both the spectrophotometric and potentiometric methods. Ferric ions form only the ML type complex with this ligand. The stoichiometry of the complex was also determined by Job's method.