Color reaction of O, O-dimethyl 2, 2-dichlorovinyl phosphate (I) and various ketones in alkaline ethanol solution was examined, and acetophenone (II) was found to be the most advantageous reagent. I was determined in insecticidal preparations. Dipterex, dibrom, and dichloroacetaldehyde (III) showed similar absortion spectra. Dipterex and III were removed by extraction. Several cormmercial preparations have been analyzed by this method with a precision of 1-2%. Mechanism of this color reaction was suggested to be dehydrative condensation reaction of II and III produced from I by hydrolysis in alkaline ethanol solution. Among various kinds of aldehydes, acetaldehydes showed characteristic absorption spectra with two intense peaks at 480 and 600 nm.