A mathematical equation, namely Jouyban-Acree model, for calculating apparent acid dissociation constants (pK(a)) in hydro-organic mixtures with respect to the concentration of organic solvent and temperature is proposed. The correlation ability of the model is evaluated by employing pKa values of 17 different acids in water-cosolvent systems. The results show that the model is able to correlate the pKa values with an overall average percentage differences (APD) of 1.71 +/- 1.86%. In order to test the prediction capability of the model, nine experimental pK(a) values from each data set have been employed to train the model, then the pK(a) values at other solvent compositions and temperatures were predicted and the overall APD obtained is 2.10 +/- 2.42%. The applicability of the extended form of Jouyban-Acree model on pK(a) data of analytes in ternary solvent mixtures is also shown. (c) 2004 Elsevier B.V. All rights reserved.