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Abstract: Using patent data from the United States, Japan, and Germany, this paper examines both innovation and diffusion of air pollution control equipment. Whereas the United States was an early adopter of stringent sulfur dioxide (SO2) standards, both Japan and Germany introduced stringent nitrogen dioxide (NO X ) standards much earlier than the US. Nonetheless, in both cases, tightened standards in the US led to more domestic patenting, but not more foreign patenting. Overall, the data suggest that inventors respond to environmental regulatory pressure in their own country, but not to foreign environmental regulations. Moreover, any technology transfer that occurs appears to be indirect. Domestic innovation occurs even for technologies that have already experienced significant innovative activity abroad and utilities purchase pollution abatement equipment from domestic firms. However, patent citation data from the US do show that earlier foreign patents are an important building block for NO X pollution control innovations in the US. [Copyright &y& Elsevier]