The acid-base properties of poly(allylamine) (MW = 15 kDa) were determined by potentiometry and calorimetry in aqueous solutions at t = 25 degreesC. Potentiometric measurements were carried out in a wide range of ionic strengths (0.1 less than or equal to I/mol.L-1 less than or equal to 5.0) in NaCl(aq) and LiCl(aq), while enthalpy changes for the protonation of poly(allylamine) were determined by calorimetry in the same ionic strength range but only in NaCl(aq). Analysis of the experimental data was carried out using two different models: the first based on a modified Henderson-Hasselbalch two parameter equation and the second on the three parameter equation proposed by Hogfeldt. Protonation constants are given for both models and ionic media at t = 25 degreesC, while DeltaG, DeltaH, and TDeltaS values were determined using the three parameter model alone. The dependence on ionic strength of protonation constants was determined by the SIT model (Specific ion Interaction Theory), while the parameters for the dependence on ionic strength of DeltaH and TDeltaS were determined by a simple empirical equation. Comparison is also made of the protonation constants of poly(allylamine) and some low molecular weight polyamines. Thermodynamic parameters for the protonation of poly(allylamine) are very close to those of low molecular weight polyamines, which are included for purposes of comparison.