A new type of organic-inorganic hybrid solid acid containing both phosphonic acid groups and sulfonic acid groups was prepared by reacting 3-sulfopropyl phosphonic acid (SP) with Zr4+, Fe(3+)or Ce(4+)ions for medium-temperature proton conduction. The molar ratio of SP to metal ions was screened to pursue high proton conductivity while ensuring the insolubility of the proton conductor (MSP) in water, which avoided leaking during PEMFC operation under hydration. The organic-inorganic structure offered good compatibility with polymer electrolyte and avoided leaching due to the insolubility in water. The proton conductor exhibited good thermal stability and high ion exchange capacity (1.96 meq. g(-1)for ZrSP(1:2)). The proton conductivity reached 0.13, 0.072 and 0.034 S cm(-1)at 100% RH, 50% RH and anhydrous conditions at 180 degrees C, respectively. The mechanism for proton conduction was deduced from the activation energy. The sulfonic acid groups and phosphonic acid groups rendered good proton conductivity via vehicle mechanism at high relative humidity (RH) and via hopping mechanism at low RH, respectively. Moreover, the flexible alkyl backbone could improve the mobility of the proton conducting groups attached on it, and thus enhance the proton conductivity. For MSP, possible applications in medium-temperature proton conduction were demonstrated. [GRAPHICS] .