Accurate prediction of epitaxial thin film properties requires complete knowledge of the chemical reaction kinetics that occurs in the gas phase and at the deposition surface. The choice of reactor operating conditions and physical designs has a significant influence on the selectivity among different reaction pathways, as is the case in GaN where two competing reaction pathways exist. The extent to which each pathway participates in the total deposition scheme is a function of reactor geometry, operating conditions, and the degree of precursor mixing as determined by the design of gas delivery systems. A detailed chemistry model is developed in this paper to study the interplay between the transport of reactants, adduct formation chemistry, and deposition kinetics within a MOVPE reactor showerhead system. This paper will demonstrate both qualitative and quantitative validation of the chemistry model to showerhead deposition experiments. (c) 2006 Elsevier B.V. All rights reserved.