We have measured the electron-stimulated desorption (ESD) of D-2, O-2, and D2O, the electron-stimulated dissociation of D2O at the D2O/Pt interface, and the total electron-stimulated sputtering in thin D2O films adsorbed on Pt(111) as a function of the D2O coverage (i.e., film thickness). Qualitatively different behavior is observed above and below a threshold coverage of similar to2 monolayers (ML). For coverages less than similar to2 ML electron irradiation results in D2O ESD and some D-2 ESD, but no detectible reactions at the water/Pt interface and no O-2 ESD. For larger coverages, electron-stimulated reactions at the water/Pt interface occur, O-2 is produced and the total electron-stimulated sputtering of the film increases. An important step in the electron-stimulated reactions is the reaction between water ions (generated by the incident electrons) and electrons trapped in the water films to form dissociative neutral molecules. However, the electron trapping depends sensitively on the water coverage: For coverages less than similar to2 ML, the electron trapping probability is low and the electrons trap preferentially at the water/vacuum interface. For larger coverages, the electron trapping increases and the electrons are trapped in the bulk of the film. We propose that the coverage dependence of the trapped electrons is responsible for the observed coverage dependence of the electron-stimulated reactions. (C) 2004 American Institute of Physics.