Measurements of oxygen permeation through dense LaCoO3-delta membranes prepared by different routes (standard ceramic or from organic precursors) showed a considerable role of processing conditions and microstructure on permeation fluxes. The higher permeability observed in ceramics produced by the ceramic route was attributed to a lower grain-boundary resistance to oxygen transport. Experimental data were also compared with results of numerical modeling of oxygen transport in LaCoO3-delta based on isotopic diffusion data in single crystals. Ionic conduction in ceramics with smaller grain size is lower than in single crystals, suggesting a significant grain boundary resistance. In contrast, oxygen permeability of LaCoO3-delta prepared by the standard ceramic synthesis route, involving higher sintering temperatures is higher than expected. This suggests easy diffusion along grain boundaries. The influence of the ceramic microstructure on total electrical conductivity and thermal expansion of lanthanum cobaltite ceramics was found negligible. (C) 2000 Elsevier Science B.V. All rights reserved.