Cobalt nanoparticles with average diameters of 8.8 nm and a standard deviation of 8% were obtained in a pressure drop induced decomposition synthesis in an autoclave. Samples were taken during the experiment and characterized with TEM. A significant narrowing of the size distribution coincident with particle growth was observed. The standard deviation decreased from 31% to 8% while the average size increased from 5.5 nm to 8.8 nm. In order to explain the experimentally observed narrowing of the size distribution, a model that accounts for the influence of the capping layer on the growth rate was developed. The transfer of the reacting monomer into the capping layer was considered to be similar to an adsorption process. A rate constant of 4 x 10(-5) cm(-5) mol(-1) s(-1) was obtained for the growth reaction, and it was thus concluded that the growth reaction proceeds under kinetic control rather than under diffusion control. (C) 2010 Elsevier Inc. All rights reserved.