The thermal behaviour of the MH₂(IO₃)₃ (M=K, Rb and NH₄) isomorphous compounds, as well as the IR and Raman spectra have been investigated as part of a study of the properties of solid protonic conductors. Detailed stoichiometries, sustained by TG, DTA, XRD and IR analysis, have been developed into the three phases. There were also evidences of the formation of the [I₃O₈]− polymeric phases. Their stabilities were associated to the different polarizing power of the H, K and Rb cations.