To use the protonic mesothermal fuel cell without humidification, mass transportation in diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]), trifluoromethanesulfuric acid (TfOH)-added [dema][TfO], and phosphoric acid (H3PO4)-added [dema][TfO] was investigated by electrochemical measurements. The diffusion coefficient and the solubility of oxygen were ca. 10−5 cm2 s−1 and ca. 10−3 M (=moldm−3), respectively. Those of hydrogen were a factor of 10 and one-tenth compared to oxygen, respectively. The permeability, which is a product of the diffusion coefficient and solubility, of oxygen and hydrogen were almost the same for the perfluoroethylenesulfuric acid membrane and the sulfuric acid solution; therefore, these values are suitable for fuel cell applications. On the other hand, a diffusion limiting current was observed for the hydrogen evolution reaction. The current corresponded to ca. 10−10 molcm−1 s−1 of the permeability, and the diffusion limiting species was the hydrogen carrier species. The TfOH addition enhanced the diffusion limiting current of [dema][TfO], and the H3PO4 addition eliminated the diffusion limit. The hydrogen bonds of H3PO4 or water-added H3PO4 might significantly enhance the transport of the hydrogen carrier species. Therefore, [dema][TfO] based materials are candidates for non-humidified mesothermal fuel cell electrolytes.