[en] The Administration has recently submitted a Clean Air Act Bill to Congress which would significantly modify the regulatory treatment of industrial hazardous air pollutants (air toxics). The adverse economic impacts of this legislation on the petroleum refining industry could be substantial. Depending on how EPA interprets the legislative language, the capital costs of compliance for the proposed bill could range from $1.3 to $15.0 billion. At the upper end of the range, costs of this order of magnitude would be over 2.5 times larger than the combined estimated cost of EPAs gasoline volatility (RVP) regulations and the proposed diesel sulfur content regulations. Potential compliance costs could be as much as $0.40 per barrel processed for large, complex refineries and as much as $0.50 per barrel for some small, simple refineries. For perspective, total refining costs, including a normal return on investment, are $4--5 per barrel. Because foreign refineries supplying the US will not be affected by the US air toxics regulations, US refineries may not be able to raise prices sufficiently to recover their compliance costs. For this reason, the air toxic legislation may put US refineries at an economic disadvantage relative to foreign competitors. Even under the best petroleum product market conditions, costs of $0.40 to $0.50 per barrel processed could reduce US Gulf refiner cash operating margins by as much as 29 percent. Under less favorable market conditions, such as the mid-80's when refiners were losing money, the hazardous air pollutant regulations could greatly increase US refiner operating losses and potentially lead to closure of some marginal refineries