The protonation behavior of poly(propylene imine) dendrimers and some related oligo amines has been measured using natural abundance N-15-NMR. The chemical shifts of the different shells of nitrogen nuclei as a function of pH have been directly interpreted as the degree of protonation for that shell. The thus obtained protonation behavior is described systematically using the Ising model which only requires values for the intrinsic protonation constants and for the pair interaction free energies between the different nitrogen nuclei. This subsequently leads to a quantitative description of the titration curves of the individual shells and to a rationalization of the microscopic and macroscopic equilibrium constants for these molecules.