We report a novel CO2-stable reduction-tolerant dual-phase oxygen transport membrane 40 wt% Nd0.6Sr0.4FeO3−δ–60 wt% Ce0.9Nd0.1O2−δ (40NSFO–60CNO), which was successfully developed by a facile one-pot EDTA–citric sol–gel method. The microstructure of the crystalline 40NSFO–60CNO phase was investigated by combined in situ X-ray diffraction (XRD), scanning electron microscopy (SEM), back scattered SEM (BSEM), and energy dispersive X-ray spectroscopy (EDXS) analyses. Oxygen permeation and long-time stability under CO2 and CH4 atmospheres were investigated. A stable oxygen flux of 0.21 cm3 min−1 cm−2 at 950 °C with undiluted CO2 as sweep gas is found which is increased to 0.48 cm3 min−1 cm−2 if the air side is coated with a porous La0.6Sr0.4CoO3−δ (LSC) layer. All the experimental results demonstrate that the 40NSFO–60CNO not only shows good reversibility of the oxygen permeation fluxes upon temperature cycling, but also good phase stability in a CO2 atmosphere and under the harsh conditions of partial oxidation of methane to synthesis gas up to 950 °C.