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Two general systems, brownmillerite-type Ba₂In₂O₅ and apatite-type silicates have been investigated for potential solid oxide fuel cell electrolyte applications. The combination of powder diffraction, NMR, TGA, Raman and AC impedance spectroscopy indicated the successful incorporation of phosphate, sulphate and silicate into the Ba₂In₂O₅ structure leading to a transition from an ordered brownmillerite-type structure to a disordered perovskite-type, which led to the conductivity enhancement below 800 °C, along with a significant protonic contribution in wet atmospheres. The CO₂ stability was also shown to be improved on doping. This oxyanion doping strategy has been extended to the analogous system, Ba₂Sc₂O₅, which resulted in samples with high conductivity and good stability towards CO₂. Neutron diffraction studies on La₉.₆Si₆O₂₆.₄ indicated that the interstitial oxide ion is located near the channel centre. Further interstitial anions could be accommodated through hydration, which led to displacement of the interstitial site away from the channel centre, with an accompanying swelling of the channel. Although long term annealing of these apatite silicates showed no apparent significant structural change, a reduction in the bulk conductivity was observed, while the grain boundary conductivity was improved, thus resulting in a small enhancement in the total conductivity below 400 °C.