In this study, migration mechanism of sulfur impurity contained in dualphase membrane (DPM) of Ce0.9Gd0.1O2-delta-SrCo0.8Fe0.1Nb0.1O3-delta during oxygen permeation process was systematically investigated. Sulfate phase was observed on the permeate side of the DPM after oxygen permeation using CO2 as sweep gas. Experimental results indicated that the migration of sulfur was driven by the chemical potential gradient of oxygen and CO2 sweeping obviously speeded up the migration rate. Theoretical calculations on the SrCoO3 suggested that Sr atoms tend to shift from subsurface to the surface even the initial surface is fully terminated by Co/O, which is beneficial to the CO2 adsorption and generates Sr- and O-vacancies. Sulfur atoms favor to occupy O-vacancies in the beginning; however, due to the formation of carbonates associated with CO2 sweeping, sulfur may hop from the O-vacancies in the bulk to the Sr vacancies on the surface, forming mixed sulfate and carbonate. In addition, the decrease of sulfur impurity leads to an increase of oxygen permeation flux, about a high and stable flux of 0.30 mL min(-1) cm(2) was achieved at 900 degrees C under air/CO2 gradient. (C) 2016 Elsevier B.V. All rights reserved.