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 cyclone robber 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 were equipped with cyclone robber systems. 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 cyclone robber system PM2.5 emission factor based on the three gins (11 total test runs) was 0.0018 kg/227-kg bale (0.0040 lb/500-lb bale). The cyclone robber system average emission factors for PM10 and total particulate were 0.012 kg/bale (0.027 lb/bale) and 0.022 kg/bale (0.048 lb/bale), respectively. The cyclone robber system PM2.5 emission rate from test averages ranged from 0.025 to 0.055 kg/h (0.055-0.12 lb/h). System average PM10 and total particulate emission factors were lower than those currently published in EPA AP-42. The ratios of cyclone robber system PM2.5 to total particulate, PM2.5 to PM10, and PM10 to total particulate were 8.3, 14.6, and 56.8%, respectively.