The site binding model is capable to capture the protonation behavior of various poly(ethylene imines) and poly(propylene imines). The main tenet of the model is that interactions between neighboring sites are taken into account, which leads to a binding isotherm that cannot be expressed in a simple algebraic form. For small molecules, on the other hand, the model reduces to the classical chemical equilibrium description of the protonation equilibria, whereby the corresponding binding constants are again influenced by the interactions between the sites. The model can be extended to other types of polyelectrolytes, although in many situations one must deal with their tacticity, which is often not known very precisely.