Growth rates for organometallic vapor-phase epitaxy of GaN and AlGaN were measured over a wide range of reactor conditions. Non-ideality in growth rates and alloy composition are clear indications of parasitic chemical reactions. These parasitic reaction pathways lead to formation of gas-phase particulates, complicating control of growth and composition. This data set is used in conjunction with a reacting-flow model to try to understand the chemistry leading to particulate formation. We have developed a relatively simple, nine-step reaction mechanism describing the Ga-precursor decomposition, Al-adduct formation and methane elimination, particle nucleation, and particle growth in the AlGaN system. Reacting-flow simulations were used to predict film growth rates and are in good agreement with our rotating-disk reactor experiments. (c) 2005 Elsevier B.V. All rights reserved.