In this paper, we investigated the interactions between poly(ethylenimine) [BPEI] and the proton by using (H(+)-glass electrode) potentiometry and titration calorimetry. The measurements were carried out in NaCl aqueous solutions, at T = 298 K in a wide range of ionic strength (up to I approximate to 5.0 mol center dot L(-1)). The protonation constants were calculated by following different approaches: (a) first, all the protonation constants were considered as true constants (considering BPEI as a simple low molecular weight tetramine); (b) second, the log K(3)(H) was considered as a linear function of $$($) over bar ($$($) over bar = protonation degree); and (c) the log K(3)(H) was considered as a nonlinear function of $$($) over bar. The difference in the standard deviation on the fit between (a) and (b) calculations is quite significant, while no further improvement is observed by using the (c) model. The protonation enthalpies were calculated from the calorimetric titrations. In these calculations, all the protonation steps of BPEI were considered as independent of $$($) over bar. The dependence of protonation constants and enthalpies on ionic strength was modeled using the Specific ion Interaction Theory (SIT).