In this study, the thermodynamic dissociation constant (pK(a)) values of tofacitinib in acetonitrile-water binary mixtures with of 25%, 30%, 35% and 45% (v/v) have been determined at 25-45 degrees C range of temperatures with reversed-phase liquid chromatography (RPLC). The chromatographic determination was achieved on a Kinetex Core-Shell EVO C18-Phenomenex (150 mm x 4.6 mm, 5 mu m) analytical column. For each case pK(a) values and retention factors of tofacitinib by taking into account the effect of the activity coefficients in hydro-organic water-acetonitrile binary mixtures have been evaluated and which obtain by SOLVER algorithm of spreadsheet program Excel to fit experimental data to the nonlinear expression derived. From these values, the thermodynamic aqueous pK(a) value of the drug was calculated by different approaches. Thermodynamic parameters standard Gibbs free energy (Delta G degrees), standard enthalpy (Delta H degrees) and standard entropy (Delta S degrees) derived from dissociation constant measurements at six different temperatures were calculated by from linearity plots of logK(a) against 1/T (van't Hoff plot). (C) 2018 Elsevier B.V. All rights reserved.