Oxidative aging alters the composition of organic aerosols over time, in turn affecting the ability of aerosols to seed cloud formation and scatter solar radiation. Here we explore the heterogeneous photooxidation of model organic particles with and without a soluble surfactant coating. Tricarballylic acid (TCA), a proxy for a-pinene oxidation products, serves as a representative small organic solute. Sodium dodecyl sulfate (SDS) was selected as the representative soluble surfactant because its surface properties have been extensively characterized. A flow reactor and aerosol mass spectrometer were used to determine the second-order reaction rate constant (k = (1.9 +/- 0.1) X 10(-11) cm(3) molecule(-1) s(-1)) and reactive uptake coefficient (gamma = 3.0) for the heterogeneous photo-oxidation of uncoated TCA particles by gas-phase OH radicals; such a high uptake coefficient implicates radical chain reactions in the oxidation mechanism. SDS dramatically slows the disappearance of TCA: when the SDS concentration approaches monolayer coverage, the rate of reaction of TCA with OH decreases by similar to 60% relative to the rate in the absence of SDS. These results indicate that small concentrations of surface-active molecules on atmospheric particles can protect organic solutes in the bulk from oxidative aging. This effect extends the environmental lifetime of dissolved pollutants.