The uptake kinetics of HOBr on NaBr and NaCl particle-film surfaces has been studied in a flow reactor coupled with a differentially pumped quadrupole mass spectrometer. Spray-deposited and aerosol-deposited salt-particle films were used for the measurement of HOBr reactivity. A rapid gas-surface reaction was observed on the NaBr surface at 250 K, and the initial uptake coefficient gamma(w), was determined to be 0.029 +/- 0.004 and 0.021 +/- 0.004 at relative humidities (RH) of 0.5 and 12.2%, respectively. gamma(w) is lower at 260 K. For the reaction on NaCl particle surfaces, gamma(w) decreases slightly from 2.0 +/- 0.3 x 10(-3) to 8.1 +/- 1.3 x 10(-4) at 250 K as RH increases from 1.5 to 18.2%. The results are interpreted in terms of the mechanism HOBr(ad) + NaX(s) --> BrX + NaOH(s), which involves the adsorption of HOBr on the surface, with the reactions occurring mainly on a NaX surface domain without water coverage at low RH. The surface morphology of the particle films was characterized by scanning electron microscopy. and the tortuosity factor, tau , of the NaCl particle film was determined to be 2.18. gamma(w) was corrected for internal surface effects to provide "true" uptake coefficients gamma(t). Typical gamma(t) values for HOBr on NaBr and NaCl at 250 K and RH of 10% are 2.5 x 10(-3) and 5 x 10(-5), respectively. Calculations suggest that the HOBr heterogeneous activation on NaBr may be competitive, if surface regeneration occurs.