Fundamental kinetics related to the film growth of InP and GaAs are systematically analyzed using numerical methods. Effects of the gas-phase reaction rate constant, the surface reaction rate constant and the mass diffusivity on a film growth rate are investigated. The rate-limiting step determining a typical profile of the growth rate in a horizontal MOCVD reactor is revealed both quantitatively and qualitatively using basic reaction models. The axial growth rate profile observed over 20 cm is divided into two regions: the upstream region where the growth rate increased monotonically and the downstream region where the rate decreased monotonically. The increase in the upstream region was caused by the gas-phase mass transport of the group III precursors from the inlet which was apart from the susceptor in the vertical direction. Both the temperature dependence of the mass diffusivity and the thermal diffusion have strong effect on the slope of the increase. The decrease in the downstream region reflects the decay of the gas-phase concentration of group V intermediates, the rate of which was limited by the gas-phase diffusion of the intermediate to the substrate. Using this, we can measure the value of the mass diffusivity of a group III intermediate by observing the decay in the growth rate in a horizontal MOCVD reactor. (C) 2003 Elsevier B.V. All rights reserved.