Fine particles were simultaneously collected at an urban, industrial and coastal site in Tianjin, China from December 28, 2006 to January 31, 2007 to investigate the chemical characteristics of major PM2.5 components during an annually occurring season of high pollution. Water-soluble inorganic ions, organic carbon (OC) and elemental carbon (EC) were analyzed. Daily average concentrations of PM2.5 ranged from 61.5-566.8 μg/m3 with an average of 223.0 μg/m3. The sum of the measured species accounted for about 60% of the PM2.5 mass with organic aerosols and sulfates being the two largest contributors. Total carbon (OC + EC) constituted 23.3%, 25.5% and 19.2%, and secondary ionic species (SO42-, NO3- and NH4+) constituted 27.6%, 19.0% and 25.9% of PM2.5 at the urban, industrial and coastal sites, respectively. The PM2.5 composition in the industrial area was influenced by local industrial emission sources. About 80% of the total carbon existed in the form of OC, and the average OC/EC ratios were 4.5, 5.6 and 4.8 at the urban, industrial and coastal sites, respectively. Secondary organic carbon (SOC), estimated by the minimum OC/EC ratio, accounted for about 37.2%~50.3% of total OC. High Cl- concentrations averaging 9.3 μg/m3 were found, which mostly originated from anthropogenic sources, rather than from sea-salt aerosols.