The compound Ba3Y4O9 is stable in dry air. However, in moist air it decomposes below 1200-decreases-C into Y2O3+BaCO3. The fast ionic conduction observed in the range 370-degrees-C < T < 600-degrees-C is shown to occur in an immobilized-liquid state of an intermediate decomposition product Ba(OH)2-BaO2-x and/or a eutectic Ba(OH)2-11 w/o BaCO3.