We have demonstrated that a protic ionic liquid, diethylmethylammonium trifluoromethanesulfonate ([dema][TfO]) functions as a proton conductor and is suitable for use as an electrolyte in H 2 /O 2 fuel cells, which can be operated at temperatures higher than 100°C under non-humidified conditions. In this study, in order to fabricate a polymer electrolyte fuel cell, matrix polymers for [dema][TfO] are explored and sulfonated polyimides (SPI), in which the sulfonic acid groups are in diethylmethylammonium form, are found to be highly compatible with [dema][TfO]. Polymer electrolyte membranes for non-humidified fuel cells are prepared by the solvent casting method using SPI and [dema][TfO]. The SPI, with an ion exchange capacity of 2.27meqg −1 , can retain four times its own weight of [dema][TfO] and produces uniform, tough, and transparent composite membranes. The composite membranes have good thermal stability (>300°C) and ionic conductivity (>10 −2 Scm −1 at 120°C when the [dema][TfO] content is higher than 67wt%) under anhydrous conditions. In the H 2 /O 2 fuel cell operation using a composite membrane without humidification, a current density higher than 240mAcm −2 is achieved with a maximum power density of 100mWcm −2 at 80°C.