A kinetic theory of the alloy growth instability with respect of fluctuations of alloy composition is developed. The growth mechanism stud ied is the step-flow growth of an alloy from the gas phase on a surface vicinal to the (001) surface of a cubic substrate. The epitaxial growth implies that the adsorbed atoms migrate on the surface during growth of each monolayer, and that their motion is "frozen" afeter the compoletion of the monolayer. The migration consists of diffusion and drift in some effective potential. This potential is a sum of a short-range contribution casued by composition fluctuations in the top completed monolayer and of a long-range elastic contribution caused by composition fouctuations in all completed monolayers. For temperatures lower than a certain critical temperature T_c, drift dominates diffusion. It results in the amplification of alloy composition fluctuations from monolayer to monolayer in the process of the alloy growth. This amplification implies that the growth of a spatially homogeneous alloy is kinetically unstable. In contrast to the effect of long-range elastic forces on the thermodynamic instability of alloys, where the elastic forces hinder the phase separation these forces favor kinetic instability and increase the critical temperature T_c.
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