In this study, the Schiff base 2-[4-Dimethylaminocinnamalamino]- benzoic acid [(CH3)(2)N . C6H4. CH: CH . N . C6H4. COOH] (DACAB) was prepared by the reaction of 2-Aminobenzoic acid with p-Dimethylaminocinnamaldehyde. The protonation constants of the ligand were determined in a 50% (v/v) dioxane - water medium using the Irving-Rossotti and PKAS-BEST computer methods.