Supercritical water (above 374.1 degrees C and 220.6 bar) is emerging as a promising medium to carry out a variety of catalytic reactions, including reforming to produce hydrogen. However, when using a heterogeneous catalyst the support material can undergo transformations in the hydrothermal environment. In this work the stability of gamma-Al2O3 modified with 1-10 wt% Ce in supercritical water is examined, specifically in the temperature range of 500-700 degrees C at 246 bar. Transformations of the gamma-phase were slowed but not prevented. Based on X-ray analysis, the transformation of gamma-Al2O3 proceeded through the K phase toward the stable a phase. Reduced cerium species were seen to be oxidized in the supercritical water environment, and low Ce-loading supports maintained the highest BET surface areas. The stabilization was greatest at 700 degrees C, where Ce-modified aluminas retained significantly higher specific surface areas than unmodified alumina. (C) 2010 Elsevier B.V. All rights reserved.