Demand for low cost and high activity catalysts for use in biomass gasification is gaining significant attention. This study investigated the effect of supercritical water gasification (SCWG) on Ni-based Al2O3 catalysts doped with La2O3 in order to gain a better understanding of the catalyst's role. The investigated micron and nano sized catalysts were characterized both before and after SCWG treatment by temperature programmed reduction (TPR) and oxidation (TPO) for determining metal-support interactions and stability. Temperature programmed desorption (CO2-TPD) was measured for CO2 adsorption to the catalyst surface. Pulse chemisorption was conducted to measure the dispersion and crystalline size of the nickel particles while XRD, TGA and Raman analysis were carried out to examine the crystalline phase and coke deposition. The results showed that exposure to SCWG severely affected the physical and chemical structure of the Ni/Al2O3 catalysts leading to agglomeration of active metals, particularly for nanostructured catalysts. Adding La2O3 to the Ni/Al2O3 catalysts retarded coke formation, especially graphitic type coke by forming lanthanum oxycarbonate species. (C) 2011 Elsevier B.V. All rights reserved.