The proton dissociation equilibria of ascorbic acid were investigated using a combination of spectrophotometric/potentiometric techniques at constant temperature 25.0 ± 0.1 °C. The study was carried out over a wide range of ionic strengths of 0.1-3.0 mol dm-3 NaClO4 in ethanol-water mixture (1:1). The Gran's method was used for glass electrode calibration to obtain pH readings based on the concentration scale (pcH). Data process for calculation of the protonation constants was done using the STAR (stability constants by absorbance reading) computer program. The dependence on ionic strength of the constants was analyzed by means of the SIT approach (specific ion interaction theory). In addition to the specific interaction parameters of the ionic species, the protonation constants at infinite dilution (zero ionic strength) were obtained. Finally, the thermodynamic functions ΔG0 (standard Gibbs free energy change) for the protonation processes were also calculated at different ionic strengths.