The effect of Au particle size, supported on TiO2, on the gas-phase hydrogenation of m-dinitrobenzene has been considered. The catalysts (Au loading = 0.1 and 1 mol%) were prepared by impregnation with HAuCl4 and a range of Au particle sizes (3.4-10.0 nm) was generated by temperature-programmed reduction over the interval 603 K <= T <= 1273 K. A thermal treatment of TiO2 at T >= 873 K was required for the allotropic change from anatase to rutile but the presence of Au lowered the requisite temperature for complete transformation by up to 400 K. m-Dinitrobenzene hydrogenation exhibited a particle size sensitivity where higher specific rates were obtained with smaller Au particles, irrespective of the support composition (i.e. anatase:rutile ratio). The reaction over each Au/TiO2 catalyst generated m-phenylenediamine (reduction of both -NO2 groups) and/or m-nitroaniline (reduction of one -NO2 group). A parallel/consecutive kinetic model has been applied to quantify the catalytic selectivity where Au particles <5 nm favoured m-nitroaniline production. The dependence of hydrogenation performance on Au particle size is accounted for in terms of a modification to Au electronic character, which impacts on m-dinitrobenzene adsorption/activation. (c) 2009 Elsevier Inc. All rights reserved.