Addition of low quantities of ytterbium to sol-gel prepared Ni/gamma-Al2O3 catalysts has been shown to lead to significant increases in catalytic activity and long term stability in the catalytic conversion of CO2 and CH4 into syngas (H-2 and CO). The role of ytterbium in these catalysts was investigated in this study through detailed investigations on the structure and composition of ytterbium promoted Ni/gamma-Al2O3 catalysts using the following techniques: synchrotron X-ray diffraction, X-ray Photoemission Spectroscopy, Transmission Electron Microscopy, Scanning Electron Microscopy/Energy Dispersive X-ray analysis, Temperature Programmed Reduction techniques and N-2 adsorption-desorption isotherms. The results obtained indicated that ytterbium, at small quantities (up to 2 wt%), interacted strongly with the support which in turn altered the interaction between nickel and the support (most notably it was found to completely inhibit the formation of NiAl2O4). This decreased interaction between Ni and the support also led to a higher quantity of Ni being present in the catalyst in the form of Ni. Copyright (C) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.