This report is part of a project to characterize cotton gin emissions from the standpoint of stack sampling. In 2006, 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 was the development of PM2.5 emission factors for cotton gin unloading systems based on the EPA-approved stack sampling methodology, Method 201A. The project plan included sampling seven cotton gins across the cotton belt. Key factors for selecting specific cotton gins included: 1) facility location (geographically diverse), 2) industry representative production capacity, 3) typical processing systems, and 4) equipped with properly designed and maintained 1D3D cyclones. Three of the seven gins had unloading systems that used pneumatic conveyance and had exhaust airstreams that were not combined with another system. In terms of capacity, the three gins were typical of the industry, averaging 24.2 bales/h during testing. Average measured unloading system PM2.5 emission factor based on the three tests (eight total test runs) was 0.022 kg/227-kg bale (0.049 lb/500-lb bale). The unloading system average emission factors for PM10 and total particulate were 0.071 kg/bale (0.157 lb/bale) and 0.120 kg/bale (0.265 lb/bale), respectively. The unloading system PM2.5 emission rate from test averages ranged from 0.35 to 0.71 kg/h (0.76-1.56 lb/h). System average PM10 emission factors were higher and system average total particulate emission factors were lower than those currently published in EPA AP-42. The ratios of unloading system PM2.5 to total particulate, PM2.5 to PM10, and PM10 to total particulate were 18.4, 31.0, and 59.4%, respectively.