This action revises the definition of volatile organic compounds (VOC) for purposes of preparing State implementation plans (SIP's) to attain the national ambient air quality standards (NAAQS) for ozone under title I of the Clean Air Act (Act) and for the Federal implementation plan for the Chicago ozone nonattainment area. This action adds acetone to the list of compounds excluded from the definition of VOC on the basis that these compounds have been determined to have negligible photochemical reactivity.