The kinetics of the heterogeneous reaction HNO3(g)+NaBr(s)<->HBr(g)+NaNO3(s) has been investigated at 296 K using a fast-flow reactor coupled to an electron-impact ionization mass spectrometer. The concentrations of HNO3 and HBr in the presence of salts were monitored mass spectrometrically, and their decay rates were used to obtain uptake coefficients. The sizes of NaBr and NaNO3 granules were measured using an optical microscope, and their specific surface areas were estimated by a well-known relationship, Sg=6/d rho(t) where d is the average diameter of the granule and rho(t) is the true density of the NaBr or NaNO3 substrate. Our observations indicate that the uptake process comprises both physical adsorption and chemical reaction. The uptake coefficients for the forward and reverse processes, after accounting for internal surfaces by means of a mathematical model of surface reaction and pore diffusion, were found to be (2.8+/-0.5)x10(-3) and (1.2.+/-0.2)x10(-2) at 296 K, respectively. The error limits represent one standard deviation precision only. The implications for atmospheric chemistry in the marine boundary layer and Arctic troposphere are discussed.