Using in situ laser light scattering, we have observed gas-phase nanoparticles formed during AlN, GaN and InN OMVPE. The response of the scattering intensity to a wide range of conditions indicates that the AIN parasitic chemistry is considerably different from the corresponding GaN and InN chemistry. A simple CVD particle-growth mechanism is introduced that can qualitatively explain the observed particle size and yields a strong residence time dependence. We also used FTIR to directly examine the reactivity of the metalorganic precursors with NH3 in the 25-300degreesC range. For trimethylaluminum/NH3 mixtures a facile CH4 elimination reaction is observed, which also produces gas-phase aminodimethylalane, i.e. Al(CH3)(2)NH2. For trimethylgallium and trimethylindium the dominant reaction is reversible adduct formation. All of the results indicate that the AIN particle-nucleation mechanism is predominately of a concerted nature, while the GaN and InN particle-nucleation mechanisms involve homogeneous pyrolysis and radical chemistry. (C) 2003 Elsevier B.V. All rights reserved.