The thermal stability of the hydrate-like Ba2(In 1-xMxIII)2O 5·nH2O, in which MIII denotes the trivalent elements Ga, Sc, Lu, and Y, was studied. When brownmillerite and its related type Ba2(In1-xMxIII) 2O5 were cooled from 873 K to room temperature in a thermogravimetry-differential thermal analysis (TG-DTA) apparatus under a wet atmosphere, the exothermal peak in DTA accompanying an abrupt mass increase attributable to the formation of hydrate-like Ba2(In 1-xMxIII)2O 5·nH2O was observed; the mass increase occurred in the temperature range of 573-673 K. For MIII = Sc, Lu, and Y, the value of n equaled 1, and the structure was the same as that of Ba 2In2O5·H2O. For M III = Ga, the hydrate-like compound of Ba2(In 1-xGax)2O5·0.6H2O (n = 0.6) appeared. The phase transformation temperature from that of Ba 2(In1-xMxIII)2O 5 into that of Ba2(In1-xMx III)2O5·nH2O for M III = Sc, Lu, and Y was ∼50 K higher than that for the unsubstituted Ba2In2O5·H2O, while it was 10-20 K lower for MIII = Ga than for Ba 2In2O5·H2O. The increase in thermal stability by ∼50 K of Ba2(In1-xM xIII)2O5·H2O with MIII = Sc, Lu, and Y was discussed in terms of the higher affinity for the hydroxyl group of the basic Sc, Lu, and Y than that of amphoteric In and Ga.