Abstract. To investigate the characteristics of PM_2.5 and its major chemical components, formation mechanisms, and geographical origins in the two megacities, Chengdu (CD) and Chongqing (CQ), in Sichuan Basin of southwest China, daily PM_2.5 samples were collected simultaneously at one urban site in each city for four consecutive seasons from autumn 2014 to summer 2015. Annual mean concentrations of PM_2.5 were 67.0 ± 43.4 and 70.9 ± 41.4 µg m⁻³ at CD and CQ, respectively. Secondary inorganic aerosol (SNA) and organic matter (OM) accounted for 41.1 and 26.1 % of PM_2.5 mass at CD, and 37.4 and 29.6 % at CQ, respectively. Seasonal variations of PM_2.5 and major chemical components were significant, usually with the highest mass concentration in winter and the lowest in summer. Daily PM_2.5 concentration exceeded the national air quality standard on 30 % of the sampling days at both sites, and most of the pollution events were at the regional scale within the basin formed under stagnant meteorological conditions. The concentrations of carbonaceous components were higher at CQ than CD, likely partially caused by emissions from the large number of motorcycles and the spraying processes used during automobile production in CQ. Heterogeneous reactions probably played an important role in the formation of SO₄²⁻, while both homogeneous and heterogeneous reactions contributed to the formation of NO₃⁻. Geographical origins of emissions sources contributing to high PM_2.5 masses at both sites were identified to be mainly distributed within the basin based on potential source contribution function (PSCF) analysis.