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 enriched in Al content relative to adjacent AlGaAs. In addition, transients are observed in the Ga desorption rate which suggest the occurrence of an Al–Ga displacement reaction. When Sb desorption is significant, GaAsSb on GaAs interfaces are found to be enriched in Sb content relative to adjacent GaAsSb. And when In desorption is significant, GaInAs on GaAs interfaces are found to be enriched in In content relative to adjacent GaInAs. The latter result is verified by x‐ray and photoluminescence measurements on separately grown, narrow, single GaInAs/GaAs quantum wells. Compositional grading for each system investigated is found to occur over a distance of about two to four monolayers. These results are shown to be consistent with a model which assumes simple first order desorption behavior and which incorporates strain‐dependent activation energies for desorption.