The kinetics of uptake of gaseous HNO3 on thin films and crystallite grains of NaCl has been studied in a coated wall tubular flow reactor coupled to a quadrupole mass spectrometer. A rapid gas-solid reaction occurs with release of gaseous HCl. Uptake coefficients were measured at room temperature with a range of partial pressures of HNO3 and H2O in the flow tube. gamma values decreased with increasing HNO3 concentration (4 x 10(11) to 700 x 10(11) molecule cm(-3)) and increased with H2O over the range 2 x 10(-4) to 10 mbar. The results were interpreted in terms of a mechanism of the reaction HNO3(g) + NaCl(s) --> HCl(g) + NaNO3(s), which involves ionization of adsorbed HNO3 molecules aided by surface-adsorbed water followed by reaction of H3O+ with Cl- and desorption of product HCl molecules. The results allow estimation of the reactive uptake coefficient for HNO3 on sea salt aerosol under atmospheric conditions.