This paper provides new estimates of efficient emission fees for sulfur dioxide (SO2) and nitrogen oxides (NOX) emissions in the U.S. electricity sector. The estimates are obtained by coupling a detailed simulation model of the U.S. electricity markets with an integrated assessment model that links changes in emissions with atmospheric transport, environmental endpoints, and valuation of impacts. Efficient fees are found by comparing incremental benefits with emission fee levels. National quantity caps that are equivalent to these fees also are computed, and found to approximate caps under consideration in the current multi-pollutant debate in the U.S. Congress and the recent proposals from the Bush administration for the electricity industry. We also explore whether regional differentiation of caps on different pollutants is likely to enhance efficiency.