The surface exchange limitations of oxygen permeation through dense mixed-conducting membranes enhance membrane stability, enabling the operation of mixed conductors, such as La0.3Sr0.7Co0.8Ga0.2O3-delta (LSCG) and La2Ni0.9Co0.1O4+delta (LNC), under air/dry CH4 gradient up to temperatures as high as 1173 - 1223 K. Testing of these materials in a model disk-shaped membrane reactor at 1023 - 1223K showed high CO2 yields (> 75%). In particular, at 1173 K, the CO selectivity was 17% for LNC and 2% for LSCG ceramics, with methane conversion efficiency of 20 and 37% respectively. Similar tendency was observed for a fuel cell-type reactor with yttria-stabilized zirconia solid electrolyte and cermet Ce0.8Gd0.2O2-delta/Pt anode, where decreasing the molar ratio between methane and electrochemically supplied oxygen from approximately 10 to 2 decreases CO/CO2 ratio at the outlet down to 0.3. This behavior suggests significant role of the complete methane oxidation on the interface between an oxygen ion-conducting membrane and gas phase, thus making it necessary to incorporate reforming catalysts in the reactors.