Abstract: A number of long-chain quaternary ammonium sites have been compared as the positively charged sites for construction of periodate ion electrodes, among them PVCelectrodes based on hexadecyltrioctylam-monium periodate showed best performance characteristics. The Nerstian response range of the electrode is from 1×10^-1 down to 6.3×10^-8Mwith an optimal working pH 2.5-7,5 and a detection limit of 1×10^-8M. In a periodate solution there exist a rather complicated equilibrium of various species of I(Ⅶ). On the basis of analysis of the pH-EMFgraph (Fig.2) it has been postulated that among various I(Ⅶ) species IO₄^- is the primary ion form that the electrode responses to. The acid dissociation constants K₁ and K₂ of H₅IO₆ have been determined potentiometrically using this postulate, the values found (K₁=5.7×10^-4;K₂ = 1.97 ×10^-7) being in good agreement with those reported in literature. The poten-tiometric selectivity coefficients of various co-existing ions have been determined.