In January 2013, a long-lasting episode of severe haze occurred in central and eastern China, and it attracted attention from all sectors of society. The process and evolution of haze pollution episodes were observed by the “Forming Mechanism and Control Strategies of Haze in China” group using an intensive aerosol and trace gases campaign that simultaneously obtained data at 11 ground-based observing sites in the CARE-China network. The characteristics and formation mechanism of haze pollution episodes were discussed. Five haze pollution episodes were identified in the Beijing-Tianjin-Hebei (Jing-Jin-Ji) area; the two most severe episodes occurred during 9–15 January and 25–31 January. During these two haze pollution episodes, the maximum hourly PM2.5 mass concentrations in Beijing were 680 and 530 μg m−3, respectively. The process and evolution of haze pollution episodes in other major cities in the Jing-Jin-Ji area, such as Shijiazhuang and Tianjin were almost the same as those observed in Beijing. The external cause of the severe haze episodes was the unusual atmospheric circulation, the depression of strong cold air activities and the very unfavorable dispersion due to geographical and meteorological conditions. However, the internal cause was the quick secondary transformation of primary gaseous pollutants to secondary aerosols, which contributed to the “explosive growth” and “sustained growth” of PM2.5. Particularly, the abnormally high amount of nitric oxide (NOx) in the haze episodes, produced by fossil fuel combustion and vehicle emissions, played a direct or indirect role in the quick secondary transformation of coal-burning sulphur dioxide (SO2) to sulphate aerosols. Furthermore, gaseous pollutants were transformed into secondary aerosols through heterogeneous reactions on the surface of fine particles, which can change the particle’s size and chemical composition. Consequently, the proportion of secondary inorganic ions, such as sulphate and nitrate, gradually increased, which enhances particle hygroscopicity and thereby accelerating formation of the haze pollution.