The formation of complexes (aggregates) between oppositely charged macromolecular species or between macromolecular species and multivalent ions is a fascinating fundamental research topic that allows one to understand fundamental processes in biology and in polymer science. In addition interpolyelectrolyte complexes hold by strong interactions and polyelectrolyte coacervates in which the stabilizing interactions are weaker find many applications in food and in colloidal science. The interactions between oppositely charged species are usually investigated as a function of intensive variables like the temperature, the pH, the ionic strength, and parameters related to the charged species themselves (molecular mass, charge density, charge distribution, and so forth). It appeared however in the past few years that the interaction kinetics is also of fundamental importance; a fast mixing of the interacting species can lead to the formation of frozen and out-of-equilibrium structures. The present investigation is aimed to study the interactions between a small polyphosphate (sodium hexametaphosphate) (HMP) and a linear polyamine (poly(allylamine hydrochloride)) (PAH) from both a thermodynamic and kinetic point of view as a function of the ionic strength (in NaCl solutions). It is found, unexpectedly, that the interaction is of biphasic nature with a first exothermic regime followed by an endothermic regime. The transition between both regimes is ionic strength independent between 10 and 2000 mM emphasizing the strong interactions between both species. It occurs at a charge ratio of about 0.4 between the number of negative and positive charges and is correlated with proton release in the exothermic regime and a proton uptake in the endothermic regime. When HMP solutions are titrated in PAH solutions the turbidity of the mixtures is not the same as that obtained during the reverse titration at a given charge ratio, emphasizing the difficulty to establish an "equilibrium" phase diagram. Finally, the difference in complex formation mechanism defines conditions to obtain a macroscopic swollen material similar to compact polyelectrolyte complexes.