The degradation of MTBE and its intermediates in aqueous media by ultrasound irradiation were conducted under oxygen, nitrogen, and argon saturated conditions. For the degradation of MTBE, tert-butyl formate (TBF), and tert-butyl alcohol (TBA) rates of degradation were similar under argon and oxygen saturation conditions. However, the rate of degradation of acetone was significantly faster under argon saturation relative to oxygen saturation. The rates of degradation were slowest under nitrogen saturation conditions. Argon had the highest thermal conductivity of the gases studied and was expected to lead to higher temperatures during the cavitation process. These higher temperatures were expected to produce a higher yield of radicals, which should enhance the degradation rates. Under oxygen saturation, the major intermediates, i.e., acetone, TBA, TBF, and methyl acetate, were identified by GC-MS and monitored by purge and trap GC. Under Ar saturation, only acetone and simple hydrocarbons with low molecular weights were detected. Measurements of the BOD and COD on irradiated samples demonstrated the toxicity of the reaction solutions decreased with increased irradiation times. This is an abstract of a paper presented at the 221st ACS National Meeting (San Diego, CA 4/1-5/2001).