The Southwest Ohio has been known for PM2.5 issues due to emissions from multiple local sources, such as industry and multimodal traffic, as well as regional impacts from sulfates, nitrates and ammonia. To better understand the speciation characteristics of PM2.5 in Southwest Ohio area, data from five monitoring sites, Taft, Hook Field, St. Bernard, Lower Price Hill, and Chase were studied for the time period of 2003 to 2013. The total concentration of PM2.5 has decreased significantly since the last decade, from 13.41 µg m–3 in 2003 to 10.55 µg m–3 in 2013. The overall PM2.5 concentration also exhibited seasonal variations, with four out of five of the highest concentrations occurring in summer, and the fifth one in winter. With various air pollution control measures (such as Cross State Air Pollution Rule Mercury and Air Toxics Standards), both the total concentrations and the speciation of PM2.5 have changed overtime. The most dominant components of PM2.5 include sulfates, organic matter (OM), and nitrates, which are 33.4%, 31.7% and 17.7% respectively (Taft data from 2009 to 2013). Elemental carbon (EC) and crustal materials, on the other hand, took much smaller portions of 2.84% and 3.61%, respectively. The highest sulfate concentrations occured in the summer, while those for nitrates occured in the winter. EC and OM followed the same seasonal trends, which peaked in the fall and summer, and were the lowest in winter. Crustal materials exhibited the lowest concentrations in winter. Comparison of the Cincinnati, (Taft site) with other major cities nearby indicated that PM2.5 in Cincinnati is higher and the fraction of sulfates is the largest. Efforts to control its emission are still necessary.