Inorganic solid acid salt Cs-2(HSO4)(H2PO4) showed high proton conductivity in the superprotonic phase, and the phase was retained on cooling even at room temperature for a long period. In the present work, we attempt partial replacement of Cs ions by ammonium ions. Incorporation of ammonium ions up to 2.3% of the total cations is succeeded. Successful incorporation of ammonium ions is confirmed by the crystal structure measured by X-ray powder diffraction, the phase transition measured by thermal analyses and phosphorus environments measured by P-31 magic-angle-spinning (MAS) NMR. Furthermore, we present H-1 and Cs-133 MAS NMR spectra and the result of H-1{P-31} rotational-echo double resonance (REDOR) experiments. The H-1 and Cs-133 MAS NMR spectra show that incorporation of ammonium ions leads to increase of disordered structures.