Quantitative evidence is presented for the importance of alkyl peroxy photochemistry in the formation of secondary organic aerosol at 254 nm. Particles were generated by extensively oxidizing dodecanoic acid with photolytically generated hydroxyl radicals in a flow cell. The resulting particles were collected and analyzed for composition, which shows a lower contribution from multiply substituted parent molecules and much more decomposition product than expected from typical low-NOx oxidation mechanisms. Studies were performed at two separate reaction times, and kinetics modeling calculations were done using theoretical work from the combustion literature to estimate the branching of the photoexcited products. Extrapolation of the ethyl peroxy radical absorption spectrum compared to actinic flux measurements also shows that the alkyl peroxy radical absorption at similar to 310 nm leads to photochemical lifetimes under pristine tropospheric conditions that are comparable to predicted lifetimes from peroxyperoxy recombination reactions, particularly at higher altitudes.