We report the results of X-ray and neutron scattering examination of the compound Ba3Y4O9, which has been reported as an ionic conductor above 370 degrees C. We have identified that the compound is isostructural with Ba3Yb2Y2O9 and that there are no partially filled oxygen sites which might support a high ionic conductivity nor any sign of a phase transition at temperatures up to 700 degrees C. It is also shown that the observed enhanced ionic conductivity upon exposure to moist air is associated with decomposition of the Ba3Y4O9 phase. We have further demonstrated, using electron diffraction and Raman spectroscopy, that during the early stages of the decomposition there is a growth of amorphous barium hydroxides which are likely to be the source of the observed ionic conductivity.