The quantitative treatment of growth of crystals from the gas phase under diffusional limitations has been extended to chemical vapour transport, and a technologically important system, GaAs/Cl₂, is discussed in detail. Flow equations may be written for any number of species, and in general, each component may be transported by more than one species. These equations may be solved readily if it is assumed that no reactions occur between gaseous species. Growth rates have been calculated for a range of initial conditions, and the maximum growth rates for stability of the growing interface were found. The effects of reactions between gaseous species are discussed qualitatively, though they are of minor importance in the GaAs/Cl₂ system in the temperature range 800–1100 K.

A parallel is drawn between the GaAs/Cl₂ system and dissociative sublimation with an inert gas present. The role of the inert gas is played by the GaCl₃ and part of the GaCl. The amount of ‘inert gas’ present at a given temperature is dictated by the necessity for transporting chlorine back to the source end, and cannot be reduced indefinitely. While only the GaAs/Cl₂ system is discussed, the treatment may be applied to other chemical vapour transport systems.