Silicate and germanate apatites have attracted great interest because of their high ionic conductivity for fuel cells and other electrochemical applications. The values of conductivity of these materials are comparable or even higher than those of Zr0.84Y0.16O1.92 (YSZ), but vary depending on the synthetic method used and the level of densification. In order to evaluate the influence of the microstructure on the transport properties of apatite-type electrolytes, La10Si5.5Al0.5O26.75 (LSA) and La9.6Ge5.5Al0.5O26.15 (LGA) ceramics with different porosity and average grain size were prepared by varying the sintering temperature and time. Impedance spectroscopy was used to study separately both the bulk and grain boundary contributions from the overall conductivity. The bulk and grain boundary conductivities of silicates resulted highly dependent on both the porosity and the sintering temperature, on the contrary, the conductivity in germanates is barely affected by the microstructure. (C) 2013 Elsevier B.V. All rights reserved.