Fine particle samples were collected in Houston, TX every second day during the summer of 2000 as part of the EPA-sponsored Houston Fine Particulate Matter Supersite program. Nonpolar compounds including n-alkanes, polycyclic aromatic hydrocarbons (PAH), and petroleum biomarkers were quantified to determine the molecular composition of the organic fraction of ambient fine particles. The results show that n-alkanes dominate other nonpolar compound classes, with Carbon Preference Index (CPI) values ranging from 1.47 to 1.94. Although plant wax alkanes contribute to ambient concentrations of fine particles, the presence of petroleum biomarkers suggests n-alkanes mainly come from anthropogenic sources, and specific species concentrations are used to separate sites by the degree to which motor vehicle and petroleum combustion emissions impact local air quality. Concentrations of PAH indicate that combustion sources in the industrial zone of Houston contribute to ambient fine particle concentrations. Additionally, data collected during a period of widespread haze episode attributed to agricultural burning and wildfires indicate that these sources can periodically dominate other sources of fine particles in the region.