Methane was partially oxidised to CO using oxygen permeated through a I mm thick silver/Ce0.9Gd0.1O2-x (Ag/CGO) composite membrane operating at 500-700 degrees C with air at 1 bar pressure. The membranes were fabricated by sintering ultratine nanoparticles of gadolinia-doped ceria (<5 nm) coated with silver using Tollens' reaction. This unique combination led to dense composites with low content of silver (7 vol%), no reaction between the components and predominant metallic conductivity. When feeding 4% methane at 700 degrees C to a 1-mm thick Ag/CGO using Ni as reforming catalyst, the conversion reached 21% and the CO selectivity 92% with an estimated oxygen flux of 0.18 mL min(-1) cm(-2) (NTP). The samples were stable in carbon-containing atmospheres and under a large pO(2) transmembrane pressure difference at temperatures below 700 degrees C for 48 h. (C) 2015 The Authors. Published by Elsevier Ltd.