The interaction of natural (alginic and fulvic acids) and synthetic (polyacrylic acid 2.0 kDa) polyelectrolytes with some protonated polyamines [diamines: ethylendiamine, 1,4-diaminobutane (or putrescine), 1,5-diaminopentane (or cadaverine); triamines: N-(3-aminopropyl)-1,4-diaminobutane (or spermidine), diethylenetriamine; tetramine: N,N′-bis(3-aminopropyl)-1,4-diaminobutane (or spermine); pentamine: tetraethylene-pentamine; hexamine: pentaethylenehexamine] was studied at T=25°C by potentiometry and calorimetry. Measurements were performed without supporting electrolyte, in order to avoid interference, and results were reported at I=0 mol L − 1 . For all the systems, the formation of (am)L 2 H i species was found (am=amine; L=polyelectrolyte; i=1…4, depending on the amine considered). The stability of polyanion–polyammonium cation complexes is always significant, and for high-charged polycations, we observe a stability comparable to that of strong metal complexes. For example, by considering the formation reaction (am)H i +2L=(am)L 2 H i we found log K i =6.0, 6.5 and 10.8 for i=1, 2 and 3, respectively, in the system alginate–spermidine. Low and positive formation ΔH° values indicate that the main contribution to the stability is entropic in nature. The sequestering ability of polyelectrolytes toward amines was modelled by a sigmoid Boltzman type equation. Some empirical relationships between stability, charges and ΔG° and TΔS° are reported. Mean values per salt bridge of formation thermodynamic parameters (ΔX° n ) are ΔG° n =−5.8±0.4, ΔH° n =0.7±0.5 and TΔS° n =6.5±0.5kJmol − 1 for all the systems studied in this work.