In this action, EPA intends to update a regulation, appendix P to part 51, promulgated in 1975, specifying what State Implementation Plans (SIPs) must require of sources with continuous monitoring systems for four source categories under which EPA's earliest New Source Performance Standards (NSPS) required new or modified sources to have continuous monitoring systems. The four source categories are fossil fuel-fired steam generators, fluid bed catalytic cracking unit catalyst regenerators, sulfuric acid plants, and nitric acid plants. What EPA specified for SIPs in appendix P to part 51 was, by design, similar to what EPA specified in part 60 NSPS regulations at that time, including the requirement to report excess emissions on a quarterly basis, at a minimum. In 1999, EPA relaxed the reporting frequency for most NSPS from quarterly to semiannual, but didn't change the appendix P to part 51 requirements. As a result, the still-quarterly requirement under appendix P to part 51 is no longer consistent with the NSPS requirements, as originally intended. In this action, EPA would propose to change the part 51 specified required minimum reporting frequency for sources with continuous monitoring systems from quarterly to semiannual. This would allow (but not require) states to reduce, through their SIPs, the required minimum frequency of excess emissions reporting for these four source categories from quarterly to semiannual. This would harmonize the minimum reporting frequency under SIPs with what EPA requires of the same source categories in numerous other, more recent, regulations, e.g., through NSPS (part 60), NESHAP (parts 61 and 63), and the title V operating permits program (parts 70 and 71). Some States assert that quarterly reporting by the affected source categories has proven unnecessary and overly burdensome. A change from quarterly to semiannual minimum reporting frequency, although reducing burden on certain sources and States, would still provide sufficiently timely information to ensure compliance and enable adequate enforcement of applicable requirements.