The effect of different ionic media on the protonation of branched polyethylenimine (BPEI) was studied potentiometrically in aqueous solutions containing LiCl, KCl, and Me(4)NCl from (0.5 to 3) mol.L(-1) and CaCl(2) from (0.2 to 2) mol.L(-1) at T = 298.15 K. Protonation constants of BPEI in alkali metal chlorides H are quite similar, and the trend is (alkali Cl) > CaCl(2) > Me(4)NCl The dependence on ionic strength of K in different ionic media, was modeled using the Specific ion Interaction Theory (SIT) approach. Differences in protonation constants (in CaCl(2) and Me(4)NCl, at low ionic strength, I < 1 mol.L(-1)) were also interpreted in terms of weak complex formation. These complexes are quite weak: log K (M + A = MA) = (24.9 and 24) L.mol(-1), for M = Ca(2+) and Me(4)N(+) (A = BPEI), respectively, at I = 1 mol.kg(-1). BPEI protonation enthalpies and entropies were determined by direct calorimetric titrations at different ionic strengths. The Delta H(i) trend is KCl > NaCl > LiCl > CaCl(2), for each protonation step of BPEL Dependence on ionic strength for Delta H(i) was taken into account by the SIT model.