Abstract Solid inorganic–organic composites were synthesized for the application as an electrolyte in fuel cells. CsHSO4 (CHS) was mechanochemically treated with Azole, which has a five-membered nitrogen heterocyclic ring such as imidazole (Iz), 1,2,4-triazole (Tz) and benzimidazole (Bz), in dry nitrogen atmosphere to obtain composites with xCHS·(100−x)Azole (x=90–50mol%). Chemical interactions between CHS and Azoles after solid-state mechanochemical treatment were confirmed from structural observation. The proton conductivity of the composites was largely increased by introduction of Azoles, particularly in the lower temperature region than a superprotic phase-transition temperature of CHS and melting points of Azoles. Proton conductivities of 80CHS·20Iz and 80CHS·20Tz composites ranged from 7×10−4 to 2×10−3Scm−1 over a wide temperature range (60–160°C). However, CHS–Bz composites showed lower conductivities due to the lower reactivity of Bz. Proton transfer in CHS–Azole composite systems includes the proton-hopping mechanism and self-dissociation, which probably supports protic diffusion, especially in low temperature regions.