We report the effects of medium-energy (3.5 keV) electron-beam irradiation ofC60films between 1 and 4 ML thick grown on Si(100)2×1-Hstudied by high-resolution electron energy-loss spectroscopy. Electron irradiation leads primarily to molecular fragmentation. Initially, molecular fragments are discrete, and saturated with hydrogen, but continued irradiation leads to the formation of a disordered material with a graphitic local structure. Experiments performed on a single monolayer ofC60show that under irradiation, fragments can bond to the substrate via displacement or desorption of the hydrogen atoms bonded to the Si substrate. ©1998 American Institute of Physics.