Secondary organic aerosol (SOA) is a complex mixture of hundreds of semi volatile to extremely low volatility organic compounds that are chemically processed in the atmosphere, including via heterogeneous oxidation by gas phase radicals. Relative humidity (RH) has a substantial impact on particle phase, which can affect how SOA evolves in the atmosphere. In this study, SOA from dark α -pinene ozonolysis is heterogeneously aged by OH radicals in a flowtube at low and high RH. At high RH (RH = 89 %) there is substantial loss of particle volume (~ 60 %) at an equivalent atmospheric OH exposure of 3 weeks. In contrast, at low RH (RH = 25 %) there is little mass loss (< 20 %) at the same OH exposure. Mass spectra of the SOA particles were measured as a function of OH exposure using a vacuum ultraviolet aerosol mass spectrometer (VUV-AMS). The mass spectra observed at low RH overall exhibit minor changes with oxidation and negligible further changes above an OH exposure = 2 × 10 12 molecule cm −3 s, suggesting limited impact of oxidation on the particle composition. In contrast, the mass spectra observed at high RH exhibit substantial and continuous changes as a function of OH exposure. Further, at high RH clusters of peaks in the mass spectra exhibit unique decay patterns, suggesting different responses of various species to oxidation. A model of heterogeneous oxidation has been developed to understand the origin of the difference in aging between the low and high RH experiments. Differences in diffusivity of the SOA between the low and high RH experiments alone can explain the difference in compositional change but cannot explain the difference in mass loss. Instead, the difference in mass loss is attributable to RH dependent differences in the OH uptake coefficient and/or the net probability of fragmentation, with either or both larger at high RH compared to low RH. These results illustrate the important impact of relative humidity on the fate of SOA in the atmosphere.