LaAl11O18 and LaAl12O18N which adopt the magnetoplumbite type structure were prepared by high temperature sintering techniques from La2O3, alpha-Al2O3 and AlN powders. Phase purity was confirmed by powder XRD and IR spectroscopy. Impedance data for ceramic samples with Pt electrodes revealed an intra-granular ionic (La3+) conductivity with activation energies E(a)(LaAl11O18) = 2.5 eV and E(a)(LaAl12O18N) = 1.6 eV. At similar to 1540 K their conductivity is comparable to that for Sr-Li-beta-alumina with sigma similar to 3 X 10(-5) S cm(-1).