We have used a combination of experiments, reactor modeling, and quantum chemical calculations to investigate parasitic chemical reactions that occur during AlGaInN MOVPE. Our results indicate that the parasitic chemical reactions occur in the boundary layer near the growing surface and ultimately lead to the formation of nanoparticles, which we observed using in situ laser light scattering. We have also developed a thermophoretic particle capture technique to collect nanoparticles and analyze them with ex situ TEM, In all cases, the nanoparticles contained the group-III element used in the MOVPE process (i.e., Al, Ga, In). We have used FTIR to investigate the critical initial chemical reactions and conclude that the particle nucleation chemistry for AlN proceeds via a concerted pathway, while the particle nucleation chemistry for GaN and InN proceeds via a bond homolysis, or radical pathway. We have also developed a particle nucleation and growth model that explains the strong residence time dependence observed during AlN and GaN MOVPE. (c) 2006 Elsevier B.V. All rights reserved.