In the solid oxide fuel cell (SOFC) field, the presence of silicon impurities is commonly considered to be detrimental to the performance due to segregation as silica at the grain boundaries. In this paper we demonstrate, however, that silicon doping into Ba2In2O5 leads to a significant enhancement in the oxide ion conductivity. The results indicate that silicon is incorporated into the structure leading to a conversion from an ordered brownmillerite-type structure to a disordered perovskite-type, with the oxygen vacancy disordering leading to the conductivity enhancement. In wet atmospheres, the conductivity is further enhanced through a protonic contribution, leading to conductivities (2.4 x 10(-3) Scm(-1) at 400 degrees C) comparable to the best perovskite proton conductors. Thus, the results show that silicon can be incorporated into the perovskite structure, suggesting further studies in this area are warranted, particularly related to electrode materials.