Surface composition is known to influence cation and anion incorporation rates during III–V molecular‐beam epitaxy (MBE). [See, for example, Van Hove and Cohen, Appl. Phys. Lett. 47, 726 (1985).] Thus, incorporation rates are expected to vary at heterointerfaces. The details of how incorporation rates vary in time during heterointerface formation are of interest because they completely determine resulting compositional profiles and/or layer thicknesses. In this study desorption mass spectrometry is used to determine the time dependence of incorporation rates during MBE growth of III–V heterostructures, with an emphasis on the heterointerface formation process. Under many conditions incorporation rates are found to vary in a nonsteplike manner, resulting in nonsteplike compositional profiles. Three different heterointerfaces are investigated: AlGaAs on GaAs, GaInAs on GaAs, and GaAsSb on GaAs. At growth temperatures for which significant Ga desorption occurs, AlGaAs on GaAs interfaces are found to be enric...