PM_(2.5)samples were collected at urban, industrial and coastal sites in Tianjin during winter, spring and summer in 2007. Concentrations of elemental carbon (EC) and organic carbon (OC) were analyzed using the IMPROVE thermal-optical reflectance (TOR) method. Both OC and EC exhibited a clear seasonal patt with higher concentrations observed in the winter than in the spring and summer, due to cooperative effect of changes in emission rates and seasonal meteorology. The concentrations of carbonaceous species were also influenced by the local factors at different sampling sites, ranking in the order of industrial > urban > coastal during winter and spring. In the summer, the port emissions, enriched with EC, had a significant impact on carbonaceous aerosols at the coastal site. Total carbonaceous aerosol accounted for 40.0% in winter, 33.8% in spring and 31.4% in summer of PM2.5 mass. Good correlation (R = 0.84-0.93) between OC and EC indicated that they had common dominant sources of combustion such as coal bing and traffic emissions. The daily average OC/EC ratios ranged from 2.1 to 9.1, the elevated OC/EC ratios being found in the winter. The estimated secondary organic carbon (SOC) accounted for 46.9%, 35.3% and 40.2% of the total OC in the winter, spring and summer, respectively, indicating that SOC may be an important contributor to fine organic aerosol in Tianjin.