The chemical composition of total suspended particulate (TSP) matter and secondary aerosol formation have been studied during wintertime fog and haze events from urban sites (Allahabad and Hisar) in the Indo-Gangetic Plain. The atmospheric abundances of elemental carbon (EC), organic carbon (OC), water-soluble OC (WSOC) suggest that organic matter is a major component of TSP, followed by concentrations of sulphate and nitrate under varying meteorological conditions. The concentrations of EC, OC, and WSOC show a nearly 30% increase during fog and haze events at Allahabad and a marginal increase at Hisar; whereas inorganic constituents (NH4+, NO3− and SO42−) are 2–3 times higher than those during clear days at both the locations. The sulphur and nitrogen oxidation ratios (SOR and NOR) also exhibit significant increases suggesting possible enhancement of secondary formation of SO42− and NO3− during fog and haze events. The significant correlation between NH4+–SO42− (R2 = 0.66, n = 61) and an NH4+/SO42− equivalent ratio ≥ 1 during fog-haze conditions suggest near-complete neutralization of sulphuric acid by ammonia. In contrast, NH4+/SO42− equivalent ratios are less than 1 during normal days suggesting an NH3-deficient environment and the possible association of SO42− with mineral dust for neutralization. Secondary inorganic aerosol formation and their hygroscopic growth can have significant impact on atmospheric chemistry, air-quality and visibility impairment during fog-haze events over northern India.