The uniformity of epitaxial growth of GaAs by metalorganic chemical vapor deposition in a vertical‐flow rotating‐disk reactor has been investigated. Observations of the thickness of the epitaxial layer versus radial distance on the susceptor surface are made for various conditions of carrier gas flow and growth temperature. A model is proposed in consideration of hydrodynamic and thermal boundary layer effects at the susceptor surface. The effects of a parasitic processes in the limit of smaller boundary layers and transition to source input rate‐limited growth in the limit of thicker boundary layers are described.