The surface segregation of indium atoms during the growth of InGaAs/GaAs heterostructures has been investigated in situ by reflection high-energy electron diffraction (RHEED). We pointed out that strong damping of the RHEED oscillations during the deposition of InGaAs on GaAs was related to the segregation strength of indium atoms in the InGaAs layer. A simple model shows that the decay constant of the RHEED oscillations may be used to determine accurately the segregation coefficient R, as confirmed by photoluminescence (PL) measurements.