In a rotating disk reactor (1) for growing epitaxial layers on substrate (3), gas directed toward the substrates at different radial distances from the axis of rotation of the disk has substantially the same velocity. The gas directed toward portions of the disk remote from the axis (10a) may include a higher concentration of a reactant gas (4) than the gas directed toward portions of the disk close to the axis (10d), so that portions of the substrate surfaces at different distances from the axis (14) receive substantially the same amount of reactant gas (4) per unit area. A desirable flow pattern is achieved within the reactor while permitting uniform deposition and growth of epitaxial layers on the substrate.