The different effects of cucurbit[7]uril (CB[7]) on the Schiff base reactions in aqueous solution were explored by 1H NMR spectroscopy and single X-ray crystallography. With CB[7], the condensation reaction of aldehyde and primary amine is dramatically inhibited. In contrast, the presence of CB[7] does tremendously stabilize iminium cation in water through ion-dipole interactions. A single crystal structure of the complex of iminium ion 7 with CB[7] grown in water is reported.