This paper describes to what extent Ni particle size affects the sulfur-tolerance of ceria-zirconia supported Ni catalysts during autothermal reforming (ATR) of isooctane. Particle size was isolated as an experimental variable by preparing catalysts with a range of Ni loadings that had nearly identical Ni surface areas. Under sulfur-free conditions, isooctane conversion and synthesis gas yield increased as the Ni particle size increased, contrary to the expectation that smaller particle sizes with lower Ni coordination would be more active. However, larger Ni particles proved to be more vulnerable to sulfur poisoning. The poor ATR activity of small Ni particles can be attributed either to a lack of sufficiently large nickel surface ensembles, or to their higher propensity to form nickel oxides under reaction conditions. This contribution suggests that, under typical ATR conditions, more highly dispersed Ni catalysts will not result in elevated sulfur tolerance. (C) 2011 Elsevier B.V. All rights reserved.