pK 1 and pK 2 values at zero ionic strength of some dicarboxylic acids (succinic, tartaric, malonic, malic and maleic) in 20% (w/w) acetonitrile-water at temperatures ranging from 30°C to 60°C have been determined using a precise EMF method. The thermodynamic parameters H°, G°and S°of the first and the second dissociation reactions are calculated from well known equations and discussed in terms of solute-solvent interactions. An extrathermodynamic analysis of the obtained results has been performed and indicates that these dissociation reactions are entropycontrolled reactions where solute-solvent interaction plays an important role.