The acid dissociation constants of 2,2',6',2"-terpyridine were determined in aqueous + organic solvents mixtures. The organic solvents were methanol, ethanol, acetone, dimethyl formamide (DMF), and dimethyl sulfoxide (DMSO). The ionization constants of terpyridine depend largely on both the proportion and nature of the organic cosolvent. Solvent properties play a major, but not exclusive, role in the ionization of the protonated terpyridine molecule. Basicity and stabilization of the different species existing in equilibrium through hydrogen bonding together with ion-solvent interaction play an important role in the acid dissociation in the presence of organic solvent. The overall dissociation constant (pK(a1)+ pK(a2)) of the protonated terpyridine increase in the various solvents according to the following order: methanol < ethanol < acetone < dimethyl formamide < dimethyl sulfoxide.