Organic matter is one of the major components of the fine particles in Houston. In the present study, fine particulate samples collected at three sites (plus one temporary site) in Houston have been investigated to determine the molecular composition and seasonal variation patterns of individual organic compounds, including n-alkanes, PAH compounds, petroleum biomarkers, alkanoic and alkenoic acids, dicarboxylic acids and levoglucosan. Next, chemical mass balance (CMB) model has been applied to calculate quantitative contributions from major primary emission sources to ambient PM2.5 levels. These emission sources include diesel engine trucks, gasoline powered vehicles, wood combustion, paved road dust, food preparation process, vegetative detritus and natural gas combustion. Source profiles suitable for the present CMB model have been developed based on source test results published in literature and our road dust resuspension experiment. Source apportionment shows diesel and gasoline vehicle exhaust to be the major primary contributors to ambient PM2.5 collected in Houston. Moreover, 52% of annual average PM2.5 mass measured in the present study can be attributed to secondary formation in the atmosphere.