The reactive uptake of Cl and Br atoms by closely packed organic thin films was studied in a flow reactor. For Cl, the reactive uptake coefficient, &ggr;, was near collision rate for alkane and alkene surfaces. For Br, &ggr;=(3¿1) ¿ 10-2 for alkane and &ggr;=(5¿2) ¿ 10-2 for alkene surfaces. The processing of the surface was monitored using FTIR, XPS and contact angle measurements. Oxidized surface-bound products and a concurrent increase in hydrophilicity were observed. The probability of a reactive collision between Br, Cl, O(3P), O3 and NO3 and surface-bound organics is compared with that of comparable gas-phase reactions, showing that reactions with a high activation energy in the gas-phase have an enhanced surface reaction probability. The uptake coefficients for these tropospheric oxidants are used to estimate the processing time for an organic coated aerosol. ¿ 2001 American Geophysical Union