This work investigates the catalytic consequences of Ru cluster size in the Fischer-Tropsch Synthesis (FTS). Ru/Al(2)O(3) catalysts with different metal particles size have been obtained by treating the solid in pure H(2) at increasing temperatures and times. Steady-state isotopic transient kinetic analysis (SSITKA) has been carried out at 523 K, 5.5 kPa CO, 55 kPa H(2), and 124.5 kPa inert in order to determine surface residence times and coverage of reversibly bonded CO and CH(x) intermediates as a function of Ru particle size (4-23 nm). We have found that FTS with Ru-based catalysts is a highly structure-sensitive reaction when Ru < 10 nm. In this range, turnover frequency of CO consumption (TOF(CO)) increases as the particle size increases, reaching a constant value for Ru particles larger than 10 nm. The lower intrinsic activity shown by Ru clusters <10 nm may be related to the stronger CO adsorption and concomitant partial blocking of active sites, as suggested by the decreased CO surface residence time as the Ru cluster size increases in the range below 10 nm. (C) 2011 Elsevier Inc. All rights reserved.