The degradation of ten trichothecene mycotoxins by aqueous ozone was monitored by liquid chromatography-ultraviolet-mass spectrometry (LC-UV-MS). Saturated aqueous ozone ( approximately 25 ppm) degraded these mycotoxins to materials that were not detected by UV or MS. At lower levels (approximately 0.25 ppm) of aqueous ozone, intermediate products were observed. On the basis of UV and MS data, it is proposed that the degradation begins with attack of ozone at the C9-10 double bond with the net addition of two atoms of oxygen. The remainder of the molecule appears to have been left unaltered. The oxidation state at the allylic carbon 8 position had a significant effect on the ease of reaction, as determined by moles of ozone required to effect oxidation. The amount of ozone required to effect oxidation to intermediate products and subsequent degradation followed the series allylic methylene (no oxygen) < hydroxyl (or ester) < keto. Ozonation was also sensitive to pH. At pH 4-6, all mycotoxins studied degraded readily; at pH 7-8 the degree of reactivity was dependent upon the carbon 8 oxidation state; at pH 9, there was little or no reaction. Structures for some of the intermediate products are proposed.