The dependence of oxygen permeability of dense La0.3Sr0.7CoO3-delta ceramics on membrane thickness indicates significant surface exchange limitations to the permeation fluxes, which suggests a possibility to increase membrane performance by surface activation. The cobaltite membranes with various porous layers applied onto the permeate-side surface were tested at 850-1120 K, Silver-modified La0.3Sr0.7CoO3-delta membranes showed enhanced permeation at temperatures above 950 K; deposition of porous layers of PrOx and Pr0.7Sr0.3CoO3-delta had no positive effect. The maximum oxygen permeability at. These results 850-1120 K was observed in the case of porous La0.3Sr0.7CoO3-delta layers with surface density about 10 mg cm(-2) suggest that the surface exchange of lanthanum-strontium cobaltite membranes under an oxygen chemical potential gradient is limited by both oxygen sorption at the surface and ion diffusion through the surface oxide layers. Oxygen permeability of La0.3Sr0.7CoO3-delta ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.