Surface charging and electron trapping in ultrathin (1.6 nm) SiO2 films on n-type silicon during bombardment by 350-600 eV electrons are observed by electric-field-induced optical second harmonic generation (SHG). Transient surface charging by fast dissipating electrons (< 1 ms charge/discharge time) can be distinguished from oxide electron trapping occurring over hundreds of seconds. The maximum SHG enhancement corresponds to an areal density of trapped electrons of similar to 6 x 10(12) cm(-2). The gradual recovery of the SHG following electron bombardment suggests the trap sites are "slow traps", i.e., oxide traps which discharge via tunneling to the Si/SiO2 interface. The effective trap lifetime is about 500 s.