We measured (at the molecular level) the 125 organic aerosol (OA) compounds present in Chengdu in winter. OA was dominated by fatty acids, phthalate esters, and anhydrosugars, and it was deeply influenced by anthropogenic sources. As pollution worsened, secondary inorganic species and secondary organic carbon (OC) dominated the increase in PM_2.5, fatty acids and anhydrosugars dominated the increase in OA, and the contributions of secondary formation and biomass burning to OC increased markedly.