In this study, nanostructured niobia particles supported on carbon supports (carbon black and carbon nanotubes) were prepared by two strategies: impregnation and deposition precipitation (DP). We compared the hydrothermal stability of these solid acid catalysts in the aqueous phase. At 15 bar and 200 degrees C, large niobia particles were formed due to crystallization on niobia/carbon composites prepared by impregnation (Nb/CB-I and Nb/CNT-I). In contrast, no large particles were formed on niobia/carbon composites prepared by DP (Nb/CB-DP and Nb/CNT-DP). For the aqueous phase dehydration of 2-butanol at 52 bar and 240 degrees C, deactivation was observed on the Nb/CB-I catalyst due to the formation of large niobia particles. The niobia/carbon catalyst prepared by DP (Nb/CB-DP) exhibited high catalytic activity, which was stable for over 40 h. The conversion of gamma-valerolactone to pentanoic acid in the aqueous phase was carried out at 35 bar and 300 degrees C to investigate the stability of Pd nanoparticles on Pd/Nb/carbon bifunctional catalysts. The Pd nanoparticles were hydrothermally stable on the niobia/carbon composite prepared by DP, whereas the Pd nanoparticles sintered and were encapsulated by the niobia on the Pd/Nb2O5/CB-I. We conclude that the niobia/carbon composites prepared by DP method are much more hydrothermally stable due to the formation of nanosized niobia particles and a strong interaction between niobia and carbon. We present a roadmap for retaining oxide functionality and acidic properties under aqueous phase conditions which will be useful for future work in biomass-related aqueous reactions. (C) 2013 Elsevier B.V. All rights reserved.