The equilibria between the periodate anions in aqueous alkaline solution have been studied by ultraviolet spectrophotometry. Equilibrium constants for the formation of the dimeric periodate ion have been evaluated in the range pH 8–12, and at temperatures from 1 to 70°. The second and third dissociation constants of periodic acid have been determined. Raman and infrared spectra show that the dimeric ion is H₄I₂O₁₀^4–. The monomeric periodate ions in alkaline solution are probably H₃IO₆^2– and H₂IO₆^3–.