Composite proton-conducting materials containing mordenite and micro-particulate hydrous Sb2O3 have been prepared. Materials characterisation employed X-ray powder diffractometry (XRD), Si-29 and Al-27 MAS-NMR, Sb-121 Mossbauer spectroscopy and a.c. conductivity studies. Sb2O3-mordenite composites show conductivity enhancements (relative to parent Na mordenite) similar to those reported for ’tin mordenite’; components in both systems comprise a hydrous oxide and de-aluminated zeolite. Both XRD and MAS-NMR reveal de-alumination/disruption of the framework of the zeolite to be less extensive than in previously investigated composites of antimonic acid and mordenite, which did not exhibit conductivity enhancements.