Five horizontal flow experiments were conducted in homogeneous and layered sand packs to evaluate the effectiveness of mobilization of trichloroethene (TCE) by ethanol flooding. A small quantity (5--7 mL) of pure TCE was injected into the center of each sand pack, and then horizontal ethanol floods were conducted at darcy velocities between 4 and 12 m d-1. Once ethanol contacted the region initially affected by TCE in uniform sand packs, rapid downward and upgradient flow of TCE occurred along sloping ethanol-water interfaces. In layered sand packs the bulk mobilization of TCE appeared to be associated more with phenomena resulting from interfacial tension reduction rather than with enhanced dissolution processes. Heterogeneities promoted a variety of bypassing and mixing phenomena that resulted in, but were not limited to, physical displacement of TCE at the edges of discrete fine-grained lenses due to localized confined flow conditions; upward migration of ethanol through finer-grained soils due to buoyancy; rapid downward mobilization of TCE; upgradient mobilization of TCE around heterogeneities; and migration of TCE into finer-grained soils at locations both upgradient and downgradient of the TCE source zone. Enhanced TCE dissolution was also observed. ¿ 1999 American Geophysical Union