A study on the acid-base properties and solubility of two amino acids, namely in-Tyrosine and DL-Tryptophan, was carried out in different experimental conditions (ionic medium, ionic strength and temperature). The protonation of in-Tyrosine and in-Tryptophan was investigated in both (CH3)(4)NCl and NaCl aqueous solutions by potentiometry (ISE-[H+] glass electrode) in different ionic strength ranges (up to similar to 3.0 mol L-1 in (CH3)(4)NCl and similar to 5 mol L-1 in NaCl). Measurements in NaCl were also performed by spectrophotometry at three different temperatures (T=293.15, 298.15 and 308.15K). The potentiometric and spectrophotometric data were analyzed altogether in order to obtain reliable protonation constants. Solubility investigations were performed only at T=298.15 K, in NaCl and (CH3)(4)NCl aqueous solutions in the ionic strength range investigated for the protonation constants, with the aim to determine the Setschenow coefficient and the activity coefficients of the neutral species of the two amino acids. The dependence of protonation constants on ionic strength was modelled by means of a Debye-Huckel type equation and the SIT approach. The temperature coefficients, in NaCl aqueous solutions, were calculated by using the protonation constants at different temperatures. As a further validation of the data, for the in-Tyrosine, the Delta H values were also determined by isoperibol calorimetric titrations, at T= 298.15 K, I=0.15 and 0.5 mol L-1; these values were compared with the Delta H calculated by using the temperature coefficients. (C) 2011 Elsevier B.V. All rights reserved.