Several alumina-supported rhodium-based catalysts have been prepared using the microemulsion technology. The change of microemulsion properties led to catalysts with different Rh particle size and narrow size distribution while keeping the same metal loading. The metal particle size was determined by XRD and TEM. These catalysts were tested in the CO hydrogenation reaction in order to investigate the influence of the Rh particle size. The catalytic results indicated that turnover frequency (TOF) increased about four times when the Rh particle size increased approximately from <5 to 30 nm. Product selectivity also depended on particle size. As Rh particle size decreased, methane and oxygenated compounds formation was favored at expenses of the higher hydrocarbons (C2+). XPS shows that the electronic metal-support interaction increases as the particle size decreases, leading to partially oxidized Rh atoms. These species are responsible of oxygenate formation, and as a result, its selectivity increases as rhodium particle size decreases. (C) 2004 Elsevier B.V. All rights reserved.