Desorption Mass Spectrometry (DMS) is used to monitor In desorption kinetics at the InAs/GaAs (001) heterointerface. In incorporation is determined to be significantly affected by temperature, surface reconstruction, and the V/III flux ratio. It is shown that the onset of the As-stabilized (2×4)-to-In-stabilized (4×2) surface phase transition is surface stoichiometry dependent, and that both the As-stabilized and the In-stabilized surface exhibit first order desorption behavior. Desorption activation energy of In from the (2×4) and the (4×2) surface is measured. In addition to an In-stabilized regime, an In-accumulated regime is identified by its deviation from first order desorption behavior.