Diluted exhaust from a diesel engine was photo-oxidized in a smog chamber to investigate secondary organic aerosol (SOA) production. Photochemical aging rapidly produces significant SOA, almost doubling the organic aerosol contribution of primary emissions after several hours of processing at atmospherically relevant hydroxyl radical concentrations. Less than 10% of the SOA mass can be explained using a SOA model and the measured oxidation of known precursors such as light aromatics. However, the ultimate yield of SOA is uncertain because it is sensitive to treatment of particle and vapor losses to the chamber walls. Mass spectra from an aerosol mass spectrometer (AMS) reveal that the organic aerosol becomes progressively more oxidized throughout the experiments, consistent with sustained, multi-generational production. The data provide strong evidence that the oxidation of a wide array of precursors that are currently not accounted for in existing models contributes to ambient SOA formation.