With its rapidly growing population and large industrial base, the megacity of Karachi, Pakistan, has been subjected to an increasing amount of ambient particulate matter (PM). Fine particulate matter (PM2.5) in Karachi was collected every 24 hours from January 8 to January 29, 2006. The samples were extracted and analyzed by gas chromatography-mass spectrometry (GC-MS) and ultra-performance liquid chromatography (UPLC) coupled with a triple quadrupole mass spectrometric detector (TQD) and time-of-flight (TOF) mass spectrometer. The daily PM2.5 levels ranged from 99.5 to 251.1 µg m–3 with a mean of 177.4 µg m–3, averaging 4–10-fold higher than the WHO guideline for 24-hour averaged PM2.5 (25 µg m–3). We found that the day-of-the-week variations of PM2.5 demonstrate Sundays have significantly lower concentrations (with a t-based confidence level of 95%), indicating that weekly behavioral patterns affect local PM2.5 concentrations. A significant negative correlation was found between the daily concentrations of levoglucosan, a biomass burning tracer, and the average daily temperatures (r = –0.589, p = 0.004), implicating heating as a major source of biomass burning emissions. Results indicate that polyaromatic hydrocarbons, hopanes, steranes, and alkanes are mainly emitted from fossil fuels and the combustion of carbonaceous materials. Organosulfates (OSs) and sulfonates were also quantified; significant correlations between OSs and sulfonates indicate a common source and/or similar formation mechanisms, while correlations with hopanes, steranes, and levoglucosan suggest their emission from primary sources such as fossil fuels and biomass burning. Qualitative analysis suggests the presence of a C6–C9 alkyl sulfate series. Through this study, the chemical composition and origin of the organic fraction of PM2.5 in Karachi has been evaluated for the first time, and the results indicate a strong anthropogenic influence on combustion emissions, particularly those from biomass and fossil fuel burning.