A year-long offline based study was performed at five sites of Indo-Gangetic plain to get the spatio-temporal variation in elemental carbon (EC), organic carbon (OC), water-soluble organic carbon (WSOC) and secondary organic aerosol (SOA) in PM2.5 fraction. OC constitutes ~70–90% of the total carbon with high water-soluble fraction (60–70%) during winter, except at Hisar. The change in concentration of OC was insignificant during summer, whereas the water-soluble fraction increased by 75–85%. The relatively high OC/EC ratio (7–11) and OC/K+ ratio (10–52) during winter and post-monsoon suggest the dominance of biomass burning from wood fuel and agricultural waste. EC is nearly constant over the seasons at all sites suggesting the dominant presence of traffic-originated EC throughout the year. Interestingly, a low correlation (~0.3) between OC and EC at IITD and IITMD indicates their source from different origins. Exceptionally, the daytime OC/EC ratio was slightly higher than night time during winter at Hisar. However, other sites showed the opposite trend. A significant linear relationship between WSOC and secondary organic carbon suggests dominant contribution from SOA. A paired t-test results indicate much of WSOC is of secondary origin and regional in nature. The secondary fraction contributed 30–60% of OC during winter across all the sites in IGP, whereas during summer it varied between 20 and 40%. SOA estimated by the EC- tracer method has the highest contribution at IITMD (~138 μg/m3) among all the sites and peaked during post-monsoon. © 2020 Turkish National Committee for Air Pollution Research and Control