Stepwise protonation constants of two purine nucleosides (adenosine and guanosine) were determined at different temperatures (293.15 to 308.15) and various ionic strengths (0.101 to 3.503 mol.kg(-1) NaClO4) using a combination of potentiometric and spectrophotometric method. The thermodynamic parameters (i.e. enthalpy change, Delta H, and entropy change, Delta S) of the protonations were calculated at different temperatures using van't Hoff and virial equations. The dependence of the protonation constant on ionic strength is modeled by a Debye-Hukel type equation andiscussed. Finally, the protonation constants of the nucleosides and the enthalpy change of protonations were determined at zero ionic strength. (C) 2014 Elsevier Ltd. All rights reserved.