The oxygen-ionic conductivity of isovalent-doped complex oxides characterized by the Ruddlesden–Popper structure is studied. The BaLa0.9Nd0.1InO4 sample was obtained for the first time by substitution in the La sublattice, and its transport properties are studied. A comparing of the results with the data for samples obtained earlier by isovalent substitution in the In-sublattice of BaLaInO4 is presented. The introducing of a dopant is shown to lead to increase in the contribution from oxygen-ionic conductivity and also in the total conductivity by ~2 orders of magnitude. © 2023, Pleiades Publishing, Ltd.