High temperature/pressure in situ Attenuated Total Reflection Infrared (ATR-IR) spectroscopy was used to investigate the phase transformation of support γ-Al2O3 into boehmite (AlO(OH)) under the hydrothermal conditions of aqueous phase reforming (APR). Activation energy barriers of boehmite formation in hot compressed water at temperatures between 150 and 180 °C were calculated to be 15.9 ± 4.8 kJ mol−1 for γ-Al2O3 and 43.2 ± 4.3 kJ mol−1 for Pt/γ-Al2O3. The influence of Pt particles is suggested to slow down the phase transformation by selective blockage of the surface nucleation sites. The presence of ethylene glycol has also an inhibiting effect on the transformation due to the carbon deposits formed on the oxide surface. Post-mortem analysis using Raman spectroscopy, 1H and 27Al MAS NMR confirms the formation of boehmite.