Protonation constants of salicylideneaniline and methyl-, ethyl-, methoxy- fluoro-, chloro-, bromo- and iodo- substituted salicylideneanilines were determined by spectrophotomeric titrimetry in 10, 30, 50 and 70% ethanol-water mixtures at constant ionic strength and at 25degreesC. The results obtained from this work were compared with those obtained using the potentimetric method. The difference between the results found with these two methods was approximately +/- 0.20 logK units. The spectrophotometric method was judged to be more suitable than the potentiometric method for halogen substituted salicylideneanilines with the low logKvalues and in the solvent mixture containing 10% ethanol, due to the low solubility of Schiff's bases.