The following is being submitted under the provisions of the Toxic Substances Control Act 90 CFR 2029; 15 U.S.C. 2607. We have found that small amounts of methyl vinyl ketone (MVK: CAS No. 78-94-4: RTECS No. 21978) are being formed in the combustion zone of a plant incinerator burning waste methyl ethyl ketone (MEK: CAS No 79-93-3; RTECS No. 21754) from a plant process. According to RTECS the 4-hour LC50 for rats for MVK is approximately 2.5 ppm. The odor threshold is about 200 ppb. Our measurements indicate that the MVK concentration exit the incinerator combustion area (1600 deg F) is approximately 25 ppm. This is reduced to approximately 2 ppm by a secondary catalytic combustion unit. With the topographic and meteorological conditions at this plant, an EPA-recognized conservative downwash computer model predicts an 8-hour average ground level concentration of 1 ppb at the point of nearest human habitation. We will check this with ground sampling when the process starts again in January, 1989. We also plan to evaluate the effect of changing combustion parameters on MVK production.