The sintering of alumina-supported nickel particles has been studied after heat-treatment in ammonia + hydrogen at 523 K and 250 bar. The investigated samples were nickel supported on gamma-alumina, transalumina and alpha-alumina, and co-precipitated nickel oxide-alumina. The sintering process was mainly followed by hydrogen chemisorption. The samples were also characterised by specific surface area measurements, X-ray diffraction, temperature programmed desorption of ammonia, in situ FTIR spectroscopy and temperature programmed reduction. For nickel supported on gamma-alumina, up to 40% of the initial metal surface area remained after the heat-treatment in ammonia + hydrogen compared with alpha-alumina or transalumina where only 10-20% of the initial metal surface area remained after the heat-treatment. The sintering can be correlated to the bond strength between the metal particle and the support. The larger the number of low-coordinated surface aluminium sites is, as for gamma-alumina, the stronger the metal-support interaction is and this in turn suppresses diffusion of nickel particles and/or atoms. (c) 2006 Elsevier B.V. All rights reserved.