Kinetic information on the substitution of sea-salt chloride by nitrate was deduced from a smog chamber investigation bn the reaction of airborne NaCl with HNO3. It was found that a measurable reaction only occurred when the NaCl-particles were present in the form of droplets. The substitution of chloride by nitrate was independent of size, which shows that the generation of the product (HCl gas) was the rare-limiting reaction step. The rate of this reaction was more than an order of magnitude slower than the rate at which nitric acid can reach the droplets. The substitution of chloride by sulphate, in a reaction between H2SO4 and NaCl, depended on particle size from which it was concluded that the transport of H2SO4 to the aerosol was the rate-limiting process. The difference in reaction of the two acids is explained by tie fact thar sulphuric acid is a condensable species, whereas nitric acid is a gas. From the amount of sulphate as a function of size an uptake coefficient for the condensing sulphuric acid was deduced as 0.1 or higher.