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In a region heavily influenced by anthropogenic and biogenic atmospheric emissions, recent field measurements have attributed one-third of urban organic aerosol by mass to isoprene epoxydiols (IEPOX). These aerosols arise from the gas-phase oxidation of isoprene, the formation of IEPOX, the reactive uptake of IEPOX by particles, and finally the formation of new compounds in the aerosol phase. Using a continental-scale chemical transport model, we find a strong temporal correspondence between the simulated formation of IEPOX-derived organic aerosol and these measurements. However, because only a subset of isoprene-derived aerosol compounds have been specifically identified in laboratory studies, our simulation of known IEPOX-derived organic aerosol compounds predicts a mass 10-fold lower than the field measurements, despite abundant gas-phase IEPOX. Sensitivity studies suggest that increasing the effective IEPOX uptake coefficient and including aerosol-phase reactions that lead to the addition of functional groups could increase the simulated IEPOX-derived aerosol mass and account for the difference between the field measurements and modeling results.