Potentiometric and calorimetric methods were used to determine the protonation constants and thermodynamic values of the protonation of different noncyclic 1,5-diamines, noncyclic 1,8-diamines and cyclic 1,8-diamines in aqueous solution. The ligands were modified by oxygen and sulfur atoms at the 3- and 6-position. The measured values of the stability constants and reaction enthalpies are compiled with data from the literature. The protonation constants of the alkyl diamines exceed those of the alkyl diamines containing oxygen or sulfur atoms by more than one order of magnitude. The main contribution to the protonation constants of the noncyclic ligands is the reaction enthalpy. The values of the reaction entropies of the noncyclic diamines are nearly constant and close to zero. No intramolecular stabilization of the protonated amines by hetero atoms could be observed in aqueous solution.