A two-dimensional numerical model employing the method of finite elements is described for mass transfer-controlled chemical vapor deposition in channel reactors. This model includes all relevant transport processes occurring in the two primary dimensions, including thermal diffusion, buoyancy forces, and the diffusion of energy, mass, and momentum in the axial (flow) direction. The model represents conditions occurring in reactors with large aspect ratios (channel width/height) in the absence of longitudinal roll waves, conditions conducive to the best growth uniformity. The model was used to calculate growth rate uniformity for metalorganic chemical vapor deposition (MOCVD) growth of InP from trimethyl indium. Thermal diffusion was found to be a significant term in the mass transfer equation, decreasing growth rate by about 30% near the front of the susceptor and by lesser amounts further downstream. Growth rate was found to be insensitive to susceptor temperature but dependent on the binary diffusion coefficient and thermal diffusion factor of the reactant in the carrier gas. Use of a tilted susceptor allows for conditions to be established for which there is a region of relatively uniform growth rate. Tilt angle must be chosen to balance between high growth rate and uniformity over large area.