The effective oxygen conductivities in the zirconia networks of porous LSM-YSZ and LSM-SYSZ composites [i.e. La0.85Sr0.15MnO3-(Y2O3)(0.08)(ZrO2)(0.92) and La0.85Sr0.15MnO3-Sc2O3)(0.1)(Y2O3)(0.01)(ZrO2)(0.89), respectively] were evaluated by an AC impedance technique using specimens in which LSM was removed by hydrochloric acid leaching. The oxygen conductivities of porous YSZ and SYSZ alone followed a Koh-Fortini relationship. LSM-containing zirconia network conductivities were additionally decreased by the presence of the LSM. Grain boundary resistances were not measurably increased. Network constriction resistances were estimated to have a minor effect for well-formed networks. The significant microstructural differences of the YSZ and SYSZ, when formed with and without LSM, then indicate that the significant decrease in network effective ion conductivity is likely to be attributable to an increase in network interruptions and anomalously high constriction resistances resulting from differential densification promoted by the LSM. (c) 2005 Elsevier B.V. All rights reserved.