Upon solution in water, both [hydroxy(mesyloxy)iodo]benzene and [hydroxy(tosyloxy)iodo]benzene undergo complete ionization to give the hydroxy(phenyl)iodonium ion (PhI+OH) and the corresponding sulfonate ion (RSO2O-) as fully solvated species, i.e., “free” ions. The phenyliodonium solution species do not form ion pairs with the organosulfonate ions. The hydroxy(phenyl)iodonium ion is presumed to be ligated with at least one water molecule at an apical site of the iodine(III) atom originally occupied by the sulfonate ion. In view of the relative basicities of HO- and H2O, the hydroxy ligand of the [hydroxy(aquo)iodo]benzene ion (PhI+(OH2)OH) is expected to be strongly bound and the water ligand is expected to be weakly bound to the iodine(III) center. This species has a pK A at (4.30 ± 0.05). PhI+(OH2)OH and its conjugate base are present in equilibrium with the [hydroxy(auqo)]-μ-oxodiphenyldiiodine cation (Ph(HO)I−O−I+(OH2)Ph). This μ-oxo dimer is present at significant levels even in relatively dilute solutions as the combination equilibrium constant is (540 ± 50). This dimer can be protonated, and the pK A of the conjugate acid is ≈2.5. The equilibrium constant for dimerization of [oxo(aquo)iodo]benzene (PhI+(OH2)O-), the most important monomer in acidic solutions, is ≈8.6.