Sintering is an important deactivation mechanism of nickel-based catalysts used for the production of synthesis gas and hydrogen in the steam reforming process. In this work sintering of nickel catalysts supported by aluminum oxide is studied by a combination of sulfur chemisorption, X-ray diffraction and anomalous small angle X-ray scattering (ASAXS). Size distributions of nickel particles are obtained by ASAXS without any assumption on the shape of the distribution, assuming only that the nickel particles are spherical. Specific nickel surface areas calculated from the measured size distributions agree with the surface areas measured directly by chemisorption. Combining the scattering and chemisorption results allows the sintering process to be studied in detail. This combined approach shows that sintering of a 9.5 wt.% Ni/Al2O3 catalyst at 650 degreesC is dominated by a nickel particle coalescence mechanism. At higher temperatures the nickel particle size distributions indicate that migration of nickel atoms begins to contribute to the sintering process. (C) 2004 Elsevier B.V. All rights reserved.