Real-time measurements ofGexSi1−x/Si(001)composition and segregation dynamics in Sn/Si(001) in molecular beam epitaxy are demonstrated using parallel detection reflection electron energy loss spectroscopy. Parallel detection enables quantitative acquisition of low-loss spectra in a time of<500 &mgr;sand surface composition determination inGexSi1−x/Si(001)viaGe L2,3core loss analysis to a precision of approximately 2&percent; in time of order 1 s. Segregation and trapping kinetics of monolayer thickness Sn films during Si epitaxy on Sn-covered Si(100) has also been studied using theSn M4,5core loss. ©1997 American Institute of Physics.