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Perovskite oxides exhibiting mixed ionic and electronic conductivity are very promising candidates as electrode materials for oxide ion (O-SOFC) and proton conducting (H-SOFC or PCFC) fuel cell applications. In this work, we present systematic investigations on the effect of anionic composition and modification of A-site and B-site composition on crystal structure and conductivity of Ba1−yLayFe1−xNixO3−d and Ba1−yLayFe1−xNixO3−d−z(OH)z at (y = 0, 0.05, x = 0, 0.1, 0.2, 0.3). The materials were synthesized via two approaches, solid state reaction and nebulized spray pyrolysis, respectively. Structural analysis based on X-ray diffraction showed that the perovskite modification obtained strongly depends on the oxygen partial pressure and anionic composition. The Ba1−yLayFe1−xNixO3−d compounds synthesized via solid state reaction in air and under argon were found to crystallize in hexagonal (6H) and monoclinic (3C-related) crystal systems. La doping (y = 0.05) on the A-site was found to induce a structural transition to the cubic modification (3C) for air-synthesized compounds. However, independent of the value of x and y, all Ba1−yLayFe1−xNixO3−d−z(OH)z compounds synthesized by nebulized spray pyrolysis were found to crystallize in an orthorhombic (3C-related) modification (space group type Cmcm) and are isotypic to BaFeO2.33(OH)0.33. Iodometric titration was conducted to determine anion content which indicates that the highest average B-site oxidation state of +3.69 was observed for air-synthesized La-free (y = 0) compound at x = 0.1, and of +3.06 for the La-doped (y = 0.05) and Ni-free (x = 0) NSP-synthesized water-containing compound, respectively. Overall, the impedance studies showed that conductivity increases with increasing Ni-content in air-synthesized compounds while it showed a strong dependency on the Ni-content x and water-content for NSP-synthesized compounds. The highest overall conductivities for the different synthesis methods were observed for dry BaFe0.8Ni0.2O2.83 and water-containing BaFe0.7Ni0.3O2.11(OH)0.78 and are in the order of 10−2 S cm−1 and 10−6 S cm−1, respectively, and originate from mixed valence obtained in the former and/or proton conductivity for the latter. Scanning electron microscopy was used to visualize morphology which provided evidence of well-separated spherical particle morphology for samples produced via nebulized spray pyrolysis and agglomeration in compounds produced via solid state reaction.