The protonation of some new poly (amido amines) and their nonmacromolecular model compounds has been studied in aqueous solution by potentiometric and viscosimetric techniques. Sharp basicity constants have been obtained, thus confirming the peculiar behavior of this family of polymeric bases. The behavior of themodels is similar to that of the corresponding polymers even if the former are slightly more basic. In order to rationalize the unusual behavior of poly (amido amines), 13C NMR titrations and quantum chemical computations havebeen performed on some representative poly (amido amines). The resulting model stresses the importance of the stiffening of each monomeric unit upon protonation, due to the formation of intramolecular hydrogen bridges and to electrostatic repulsion between positively charged onium ions, in determining the physicochemical properties of poly (amido amines) even in the absence of direct interactionsbetween groups belonging to different monomeric units. In fact, this stiffening could affect the properties of the whole polymer (conformational transitions) because the joints between different units are very rigid.