Anodic alumina membranes (AAM) filled with cesium hydrogen phosphate proton conductor have been tested as inorganic composite electrolyte for hydrogen–oxygen thin film (≤50μm) fuel cell (TFFC) working at low temperatures (25°C), low humidity (T gas =25°C) and low Pt loading (1mgcm−2). Single module TFFC delivering a peak power of around 15–27mWcm−2, with open circuit voltage (OCV) of about 0.9V and short circuit current density in the range 80–160mAcm−2 have been fabricated. At variance with pure solid acid electrolytes showing reproducibility problems due to the scarce mechanical resistance, the presence of porous alumina support allowed to replicate similar fuel cell performances over numerous AAM/CsH2PO4 assemblies. A scale-up process of the electrodic area has been optimized in order to increase the delivered peak power of AAM thin film fuel cell. Morphological, chemical and electrochemical studies on the alumina composite electrolyte have been carried out by means of scanning electron microscopy, X-ray diffractometry, Micro-Raman spectroscopy, DTA/DTG analysis, ac impedance spectroscopy and single fuel cell tests.