Today's action promulgates revisions to the new source performance standards (NSPS) for new, modified, and reconstructed small industrial-commercial-institutional steam generating units (40 CFR part 60, Subpart Dc) that were proposed on November 15, 1995. The revisions exclude certain small steam generating units, when conducting combustion research, from the category of small steam generating units subject to NSPS control requirements for sulfur dioxide (SO<INF>2) and particulate matter (PM). The NSPS are issued under the authority of section 111 of the Clean Air Act (CAA). Following promulgation of the NSPS, litigation was filed by Babcock and Wilcox, who repeated a concern they had expressed during the public comment period following proposal of the NSPS. That is, they had requested an exemption from the NSPS for steam generating units of 14.6 MW (50 million Btu/hr) heat input capacity or less used for combustion research based on intermittent and infrequent operation. Discussions with Babcock and Wilcox made it clear that there is a legitimate concern regarding the ability of experimental, and sometimes unpredictable, air pollution control technology to consistently meet the NSPS. This, coupled with the fact that these steam generating units provide valuable data on both the combustion process and methods of air pollution control which result in improved fuel efficiency, improved air pollution control efficiency, and less expensive air pollution control, led the EPA to provide the exemption in an effort to encourage combustion research.