This action revises EPA's 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 (FIP) for the Chicago ozone nonattainment area. This action adds perchloroethylene (perc), also known as tetrachloroethylene, to the list of compounds excluded from the definition of VOC on the basis that it has negligible photochemical reactivity. Perc is a solvent commonly used in dry cleaning, maskant operations, and degreasing operations. This rule results in more accurate assessment of ozone formation potential and will assist States in avoiding exceedances for the ozone health standard. The rule does this by causing control efforts to focus on compounds which are actual ozone precursors, rather than giving credit for control of a compound which has negligible photochemical reactivity. Perc will continue to be regulated as a hazardous air pollutant under section 112 of the Clean Air Act. EPA has already issued regulations limiting emissions of perc from dry cleaning and halogenated solvent cleaning and as a feedstock in the organic chemical manufacturing industry.