The Koser reagent, [hydroxy(tosyloxy)iodo]benzene (1; Ts = p-toluenesulfonyl), is a useful oxidizing agent for a range of organic substrates.[1] It has been reported that upon dissolution in water the l 3 -iodane undergoes complete ionization to give the hydroxy(phenyl)iodonium cation (PhI + OH).[2] The hydroxy(phenyl)iodonium ion does not form an ion pair with a tosylate ion and is presumed to be ligated with at least one water molecule at an apical site of the iodine(iii) center. This aqua(hydroxy)(phenyl)iodonium ion 2 (L = H 2 O) is highly soluble in water with a pK a value of 4.30 at 20 8C.Iodosylbenzene is a polymer bridged by secondary I···O hypervalent interactions, [3] and hence is essentially insoluble in both nonreactive organic solvents and water at near neutral pH conditions; [4] however, at pH < 2.3 iodosylbenzene is highly soluble in water, in which 2 (L = H 2 O) was also assumed to be a primary monomeric species. [2] Aqua complexes of l 3 -iodanes with a water molecule coordinated to an iodine(iii) center are unique, and there are no well-established precedents. Therefore, direct evidence supporting the proposed structure of 2 (L = H 2 O) is highly desirable. Recently, Wirth and co-workers synthesized an interesting chiral hydroxyiodonium ion 3, in which the oxygen atom of the ortho methoxymethyl group replaces the tosylate ligand of the Koser reagent with a near-linear O À I···O triad. [5] We have reported the isolation and characterization of the crown ether complex 2·BF 4 À (L = [18]crown-6) of hydroxy-(phenyl)iodonium ion. In this complex, an oxygen atom of the crown ether is tightly bound to the iodine(iii) center at the apical site and forms a T-shaped structure with linear OÀI···O hypervalent bonding.[6] Herein, we report the first synthesis