The dependence of oxygen permeability of dense La 0 . 3 Sr 0 . 7 CoO 3 - δ 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 850-1120K. Silver-modified La 0 . 3 Sr 0 . 7 CoO 3 - δ membranes showed enhanced permeation at temperatures above 950K; deposition of porous layers of PrO x and Pr 0 . 7 Sr 0 . 3 CoO 3 - δ had no positive effect. The maximum oxygen permeability at 850-1120K was observed in the case of porous La 0 . 3 Sr 0 . 7 CoO 3 - δ layers with surface density about 10mgcm - 2 . These results 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 La 0 . 3 Sr 0 . 7 CoO 3 - δ ceramics was found to increase with increasing grain size due to decreasing grain-boundary resistance to ionic transport.