The reactive uptake coefficients (&ggr;) of NO3 onto pure water and dilute solutions of NaCl, NaBr, and NaNO2 were measured using a wetted-wall flow-tube setup combined with a long-path absorption cell for the detection of NO3. The measured &ggr; values were in the range 1.5¿10-4-6¿10-3, depending on the salt concentration in the water. By measuring &ggr; as a function of salt concentration, HD0.5l for NO3 in water was determined to be (1.9¿0.4)¿10-3 M atm-1 cm s-0.5 at 273 K, assuming that the rate coefficient for the reaction of NO3 with Cl- is 2.76¿106 M-1 s-1 at 273 K. The Henry's law coefficient for NO3 in water is estimated to be 0.6¿0.3 M atm-1, assuming that the diffusion coefficient of NO3 in water is Dl=(1.0¿0.5)¿10-5 cm2 s-1. Uptake of NO3 on pure water is interpreted as due to reaction of NO3(aq) with H2O(l) to produce HNO3 and OH in the liquid phase. Implications of these findings to the chemistry of NO3 in the troposphere are also discussed. ¿ American Geophysical Union 1996