The binding energies of NH4+ to polyethers, and for comparison, to acetone molecules, were measured by pulsed high-pressure mass spectrometry. The binding energies in the polydentate complexes increase with increasing ligand size and number of available oxygen groups. However, the binding energies are smaller than in complexes with free Me(2)CO molecules, reflecting the geometrical constraints in the polydentate ligands. The binding energy of NH4+ in various polydentate complexes is similar to that of K+ and smaller than that of H3O+ by 105 +/- 12 kJ/mol (25 +/- 3 kcal/mol). Based on the H3O+ to crown ether binding energies, these relations can be used to estimate the binding energy of NH4+ and K+ to 15-crown-5 of 248 +/- 12 kl/mol (59 +/- 3 kcal/mol) and to 18-crown-6 of 296 +/- 12 kJ/mol (71 +/- 3 kcal/mol). The order of binding energies of oxygen-containing ligands to H3O+ > Na+ > NH4+ approximate to K+ is reproduced by ab initio calculations on complexes with MeOCH(2)CH(2)OMe. The amount of calculated charge transfer to the ligands follows the order of binding energies. The difference between the binding energies of H3O+ and NH4+ to oxygen-containing ligands in the gas-phase complexes is similar to the condensed-phase aqueous heats of solvation of these ions.