The temperature‐dependent behavior of the solid compositionxsof AlxGa1−xAs has systematically been studied as a function of gas phase compositionxgin an optimized horizontal metalorganic vapor phase epitaxy reactor at atmospheric pressure. Up to a temperature of 660 °C the Al incorporation is constant but slightly exceeds the Ga incorporation. Above this temperature the Al incorporation strongly increases with temperature. This behavior is most probably related to a change in growth mechanism from mass transport limited growth to a regime where the growth is controlled by thermodynamics, especially for the gallium species.