The Na+ complexes of 14 substituted aza-crown ethers, some containing amide and others containing ester functionalities in their side arms, were investigated by dynamic Na-23 NMR spectroscopy. Cation-exchange activation parameters and mechanisms between the complexes and the solvent were obtained in CD3CN. The 18-C-6 derivatives exchange the cation via a primarily bimolecular or associative mechanism, while the 15-C-5 ligands prefer the unimolecular or dissociative pathway. The presence of the side arms containing donor groups is important in the bimolecular exchange process of the 18-C-6 systems, helping to effect the removal of the bound cation while a second one approaches the ligand from the opposite side of the molecule. The amide-substituted 15-C-5 complexes with Na+ are very inert and exhibit characteristics similar to those of cryptates, with cation-exchange rates that are very slow, even at 70 degrees C. The observations suggest a very tight fit of Na+ in the cavity provided by the 15-C-5 derivatives containing donor groups in the side arms. Although these are generally less thermodynamically stable than their 18-C-6 analogues, they are kinetically more stable.