This report is part of a project to characterize cotton gin emissions from the standpoint of stack sampling. In 2006, the Environmental Protection Agency (EPA), finalized and published a more stringent standard for particulate matter with nominal diameter less than or equal to 2.5 µm (PM2.5). This created an urgent need to collect additional cotton gin emissions data to address current regulatory issues, because current EPA AP-42 cotton gin PM2.5 emission factors did not exist. The objective of this study is the development of PM2.5 emission factors for cotton gin overflow systems based on the EPA-approved stack sampling methodology, Method 201A. The project plan included sampling seven cotton gins across the Cotton Belt. Three of the seven gins had overflow systems where the exhaust airstreams were not combined with other major systems. Another sampled gin had an overflow system where the exhaust was combined with a trash handling system prior to the cyclone. In terms of capacity, the three gins were typical of the industry, averaging 27.5 bales/h during testing. Some test runs were excluded from the test averages because they failed to meet EPA Method 201A Test criteria. Also, other test runs, included in the analyses, had cotton lint fibers that collected in the ≤ 10 µm and/or ≤ 2.5 µm samples. This larger lint material can impact the reported emissions data, but EPA Method 201A does not suggest methods to account for these anomalies. Average measured overflow system PM2.5 emission factor based on the three tests (nine total test runs) was 0.0040 kg/227-kg bale (0.0088 lb/500-lb bale). The overflow system average emission factors for PM10 and total particulate were 0.018 kg/bale (0.040 lb/bale) and 0.041 kg/bale (0.090 lb/bale), respectively. The overflow system PM2.5 emission rate from test averages ranged from 0.027 to 0.21 kg/h (0.059-0.47 lb/h). System average PM10 and total particulate emission factors were higher than those currently published in EPA AP-42. The ratios of overflow system PM2.5 to total particulate, PM2.5 to PM10, and PM10 to total particulate were 9.7, 21.7, and 44.7%, respectively.