The effect of partial substitution of In by Zr, Ti and Hf in BaInO2.5 has been studied in order to improve its protonic conductivity. The isotope effect of H and Don the protonic conductivity has also been studied, which indicates that hydrogen diffusion is a rate-limiting process for the protonic conduction. Although the contents of substituting elements do not correlate well with the variation of the protonic conductivity, a 'free volume', which is usually introduced to interpret changes of the conductivity of oxide ion conductors, can do well. This suggests the free volume could be used as a figure of merit to improve the protonic conductivity in BaInO2.5 based oxides. (C) 2010 Published by Elsevier By.