One possible explanation for unexplained disappearance of vinyl chloride (VC) from what are thought to be anaerobic subsurface environments is that the environments are, in fact, not anaerobic. Rather, they might be subject to low, steady influx of oxygen, and aerobic oxidation could be occurring at extremely low oxygen concentrations. Studies were conducted with VC-oxidizing transfer cultures derived from two chloroethene-contaminated sites, as well as with microcosms constructed from sediment and groundwater from one of these sites. Oxygen was steadily delivered to the experimental systems using permeation tubes to maintain low dissolved oxygen throughout the time-course of investigation. VC oxidation was sustained at dissolved oxygen concentrations below 0.02 mg/L in the two transfer cultures, and below 0.1 mg/L in the microcosms. This supports the possibility that-at least at some sites-apparent loss of VC from what are thought to be anaerobic zones might, in fact, be due to aerobic pathways occurring under conditions of low oxygen flux (e.g., via diffusion from surrounding aerobic regions and/or from recharge events).