Abstract. The oxidation of secondary organic aerosol (SOA) is studied with mass spectra analysis of SOA formed in a Potential Aerosol Mass (PAM) chamber, a small flow-through photo-oxidation chamber with extremely high OH and ozone levels. The OH exposure from a few minutes in the PAM chamber is similar to that from days to weeks in the atmosphere. The mass spectra were measured with a Quadrupole Aerosol Mass Spectrometer (Q-AMS) for SOA formed from oxidation of α-pinene, m-xylene, p-xylene, and a mixture of the three. The organic mass fractions of m/z 44 (CO₂⁺) and m/z 43 (mainly C₂H₃O⁺), named f₄₄ and f₄₃ respectively, are used as indicators of the degree of organic aerosol (OA) oxidation that occurs as the OA mass concentration or the OH exposure are varied. The degree of oxidation is sensitive to both. For a fixed OH exposure, the degree of oxidation initially decreases rapidly and then more slowly as the OA mass concentration increases. For fixed initial precursor VOC amounts, the degree of oxidation increases linearly with OH exposure, with f₄₄ increasing and f₄₃ decreasing. In this study, the degree of SOA oxidation spans much of the range observed in the atmosphere. These results, while sensitive to the determination of f₄₄ and f₄₃, provide evidence that some characteristics of atmospheric OA oxidation can be generated in a PAM chamber. For all measurements in this study, the sum of f₄₄ and f₄₃ is 0.25 ± 0.03, so that the slope of a linear regression is approximately −1 on an f₄₄ vs. f₄₃ plot. This constancy of the sum suggests that these ions are complete proxies for organic mass in the OA studied.