There were 6 severe haze events over a large area of the Yangtze River Delta (YRD) region in January 2013. In this study, based on the hourly concentrations of trace gases and PM2.5 at 10 observation stations (8 city stations, 1 regional background station and 1 island station) during Jan. 1–31, 2013 as well as the concentrations of water-soluble ions at 5 stations (4 city stations and 1 regional background station) during Jan. 18–24, 2013 in the YRD region, the regional characteristics of the air pollutants during heavy haze episodes were investigated in combination with the atmospheric circulation patterns. The concentrations of PM2.5 on haze days were 1.6–2.4-fold higher than on clear days. The concentration of PM2.5, SO2, NO2 and CO increased significantly, with average values of 128.6, 48.5, 78.1 µg m–3 and 1.5 mg m–3 on haze days, and were 64.6, 36, 52.5 µg m–3 and 1.1 mg m–3 on clear days. The PM2.5 concentration of ten observation sites had positive correlations with CO and NO2, and had weakly negative correlations with O3. The sources of PM2.5, SO2, NO2 and CO were strong in inland cities and weak in coastal cities, and the sources of O3 were mainly from Wuxi, Suzhou and southeast of An’hui. The mass and water-soluble ion concentrations were both centralized in PM2.1 during the haze events; additionally, the NH4+, SO42– and NO3– ions were dominant, constituting 86–90.9% of the total ion concentrations in PM2.1. The mass spectra of NH4+, K+, Cl–, SO42–, F–, NO2– and NO3– had unimodal distributions. The secondary formations of sulfate dominated on haze days, and the nitrate oxidation rates were relatively high for inland cities and low for coastal cities.