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Determination of the proton dissociation constants of several arylphosphonic acid derivatives was carried out to investigate the accuracy of the Hammett equations available for this family of compounds. For the measurement of the pKa values modern, accurate methods, such as the differential potentiometric titration and NMR-pH titration were used. We found our results significantly different from the pKa values reported before (pKa1: MAE = 0.16 pKa2: MAE = 0.59). Based on our recently measured pKa values, refined Hammett equations were determined that might be used for predicting highly accurate ionization constants of newly synthesized compounds (pKa1 = 1.70–0.894σ, pKa2 = 6.92–0.934σ).Determination of the proton dissociation constants of arylphosphonic acid derivatives.Comparison of our experimental values to predicted and formerly measured pKa values.Hammett equations of proton dissociation of arylphosphonic acid derivatives were refined.