In this study the acidity constants of a series of biologically active pyridyl Schiff bases were calculated from UV-visible spectrophotometric data at (25 +/- 0.1)degrees C. The first acidity constants (pK(a1)) of the 2-[2-aza-2-(4-methyl(2-pyridly))vinyl]phenol (1), 2-[2-aza-2-(4-methyl(2-pyridly))vinyl]-6-nitrophenol (2), 2-[2-aza-2-(4-methyl (2-pyridly))vinyl] 4-nitrophenol (3), 2[2-aza-2-(4-methyl(2-pyridly))vinyl]-4,6-dinitrophenol (4), 2[2-aza-2-(6-methyl(2-pyridly))vinyl]phenol (5), 2-[2-aza-2-(6-methyl (2-pyridly))vinyl]-6-nitrophenol (6), 2{2-aza-2-(6-methyl (2-pyridly))vinyl]-4-nitrophenol (7), and 2-[2-aza-2-(6-methyl(2-pyridly))vinyl]-4,6-dinitrophenol (8) are found to be associated with the protonation of the phenolate oxygen. The second acidity constants (pK(a2)) are found to correspond to protonation of a pyridine nitrogen for molecules 5, 6, and 7 and oxoprotonation for molecules 1, 2, 3, 4, and 8. The third acidity constants (pK(a3)) are found to be associated with the protonation of a pyridine nitrogen for molecules 1, 3, 7, and 8 and amino protonation for molecules 2 and 5. For molecules 4 and 6, it is associated with oxo protonation. The contribution of the keto-amino tautomeric form was found to be considerably important.