The thermal stability of the hydrate-like Ba-2(In1-xMxIII)(2)O-5 (.) nH(2)O, in which MIII denotes the trivalent elements Ga, Sc, Lu, and Y, was studied. When brownmillerite and its related type Ba-2(In1-xMxIII)(2)O-5 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 Ba-2(In1-xMxIII)(2)O(5)(.)nH(2)O was observed; the mass increase occurred in the temperature range of 573-673 K. For M-III = Sc, Lu, and Y, the value of n equaled 1, and the structure was the same as that of (Ba2In2O5H2O)-H-.. For M-III = Ga, the hydrate-like compound of Ba-2(In1-xGax)(2)O(5)(.)0.6H(2)O (n = 0.6) appeared. The phase transformation temperature from that of Ba-2(In1-xMxIII)(2)O-5 into that of Ba-2(In1-xMxIII)(2)O(5)(.)nH(2)O for MIII = Sc, Lu, and Y was 50 K higher than that for the unsubstituted (Ba2In2O5H2O)-H-., while it was 10-20 K lower for M-III = Ga than for (Ba2In2O5H2O)-H-.. The increase in thermal stability by ∼ 50 K of Ba-2(In1-xMxIII)(2)(O5H2O)-H-. with M-III = 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. © 2005 The Electrochemical Society. All rights reserved.