The complexation of uronium perchlorate with crown ethers of different ringsizes (18–33 ring atoms) has been studied by using two-phase extraction experiments. Crown ethers with 27 or more ring atoms are the best hosts to transfer uronium salts from an aqueous phase to an organic phase. The amount of uronium perchlorate transferred was measured by coulometric titration of the stoichiometric amount of ammonia produced by enzymatic degradation of urea. The crystal and molecular structure of the 1:1 complex of uronium perchlorate with benzo-27-crown-9 has been determined by X-ray crystallography. The uronium cation is encapsulated in the crown ether cavity with all its hydrogen atoms bonded to the macrocyclic host. A 1:1 complex of uronium picrate with benzo-21-crown-7 was isolated and is assumed to be a perching complex.