The phenomenon of nucleation of solid particles from the gas phase has a number of important industrial applications. In this study, the formation of nickel nuclei from the gas phase decomposition of nickel carbonyl has been predicted using classical nucleation theory and compared with experimental results from a hot wall tube reactor. It was shown that the free energy nucleation barrier was much less significant in reality than predicted by theory. In one instance, application of the classical theory suggested that the nucleation onset temperature should be more than 250K higher than the onset temperature calculated from the volume free energy. The experimental results showed that the actual nucleation onset temperature was increased by less than 60 K. Further calculations showed that the observed primary particles were produced during a brief nucleation event, and that the major contribution to particle growth was the result of the surface reaction. (C) 2009 Elsevier Ltd. All rights reserved.