The interaction of quaternary ammonium ion guests with self-assembling hosts was examined by 1H NMR and ESI-MS experiments. The hosts consist of four identical, self-complementary subunits, reversibly joined in a capsular assembly through hydrogen bonding. Both approaches show the ammonium ions to be encapsulated, held within the hollow shell of the capsule. Competition experiments with a series of different guest ions reveal a characteristic size selectivity. The NMR and MS methods are complementary: MS easily reveals the formation of heterotetramers from different subunits that could not be determined by NMR, while NMR allowed competitive encapsulations of neutral and ionic guests that were not possible to detect with MS. These competition experiments gave a lower limit of ca. 3.6 kcal/mol for the contribution of cation- interactions involved in the encapsulation of the ionic guests.