High-pressure mass spectrometry is used to obtain the following clustering enthalpies and entropies of NH4+: With acetonitrile, AHo(n-I,n) (kcal mol-') n = 1, 27.6; n = 2, 21.2; n = 3, 14.2; n = 4, 11.7; and the corresponding ASo(n-l,n) (cal mol-! K-I) 24.2, 25.4, 17.5, and 22.2. With benzene, AHo(n-I,n) (kcal mol-!) n = 2, 17.0; n = 3, 14.2; and the corresponding ASo(n-l ,n) (cal mol-' K-I) 30.5 and 32.9. Comparison of cluster iod energies is made with the values for the corresponding species containing K+. Classical electrostatic analyses, based on "numerical experiments" and on a formal multipolar analysis, are used to probe the nature of the interactions. Morokuma and electrostatic analyses of quantum chemical calculations on K+.N H3 and NH,+.NH, provide additional insight. Finally, documentation is given by using examples from condensed-phase media: lattice and solvation energies, crystal dynamics, conductances of electrolyte solutions, magnetic resonance measurements on NH4+ solutions, and the interactions of some key biochemical species.