Ionic transport in the layered perovskite, LiLaTa2O7, and its oxynitride derivative, LiLaTa2O6.15N0.57, were examined by ac impedance spectroscopy in a N2 atmosphere. Non-Arrhenius behavior was observed when LiLaTa2O7 was heated from 30°C to 70°C with the conductivity decreasing from 10−5.9S/cm to 10−6.3S/cm, due to the heat-induced removal of interlayer water. The conductivity of dehydrated LiLaTa2O7 then increased monotonously from 10−6.3S/cm at 80°C to 10−3.7S/cm at 283°C with an activation energy (Ea) of 0.54eV. Compared to LiLaTa2O7, the oxynitride phase, LiLaTa2O6.15N0.57, exhibited much lower conductivity ranging from 10−6.7S/cm at 315°C to 10−5.5S/cm at 496°C with an Ea of 0.57eV. The distinct ionic conductivity among the similar layered perovskites, LiLaTa2O7, LiLaTa2O6.15N0.57, Li2La2/3Ta2O7, and Li2LaTa2O6N, highlights the importance of structural factors on the ionic conductivity, particularly, the site occupancy of the mobile cation, Li+, and the connectivity of the anion sublattice.