Understanding the secondary organic aerosol (SOA) formation is among most important topics in the field of aerosol research because its poor understanding leads to large uncertainty in the assessment of aerosol effects on air quality and climate. This study reports the diurnal and temporal variability in SOA formation over a site (Patiala: 30.2°N, 76.3°E, 249 m amsl) located in the Indo-Gangetic Plain (IGP) during winter using the first online measurements of water-soluble organic carbon (WSOC) over India. Online WSOC, measured with particle-into-liquid sampler (PILS) connected to total organic carbon (TOC) analyzer, ranged from 0.1 to 99 µg m–3 (avg: 15.6, sd: 9.4) with a considerable day-to-day and within the day variability, and attributed to meteorological conditions and regional sources. Diurnal trends of online WSOC suggest significant SOA formation during 7:00 to 22:00 hrs when sources of SOA precursors are active; and loss of SOA occurs during afternoon when ambient air temperature is at its peak. In parallel to online measurements, filter-based particulate matter smaller than 2.5 µm (PM2.5) samples were also collected and analyzed for major cations, anions and carbonaceous aerosols. Filter-based PM2.5 composition suggests that the emissions from biomass burning contribute more to carbonaceous aerosols than those from fossil fuel burning. In spite of this, average primary WSOC was only ~20% whereas secondary WSOC (or SOA) dominated the total WSOC concentration with ~80% contribution. A strong linear relationship between PM1 and WSOC (R2 = 0.83, slope = 0.113, intercept = 4.7), suggests that a significant fraction of fine particles are SOA.