The uptake of N2O5 by pure water and NaCl solution was studied as a function of temperature in the range from 262 to 278 K with the droplet train technique, where a highly controlled beam of droplets was exposed to N2O5 in a low-pressure flow tube reactor, and the formation of nitrate in the liquid phase was determined by ion chromatography. The uptake coefficients, gamma, for N2O5 On pure water are observed to decrease from 0.03 to 0.013 with increasing temperature. This behavior corresponds to the expected negative temperature dependence of mass accommodation leading to an enthalpy Delta H-obs = (-9.6 +/- 1.6) kcal mol(-1) and to an entropy Delta S-obs = (-43 +/- 6) cal mol(-1) K-1 for the phase transfer, corresponding to a continuous nucleation process with a critical cluster size N* of about 2.4. A significantly lower yield of nitrate than with pure water is observed in the experiments on NaCl solution (1 mol/L), indicating that nitrogen compounds (such as C1NO(2)) are formed after the uptake of N2O5 by subsequent reactions with NaCl and escape from the droplets. After correction for the known yield for the formation of ClNO2 the results exhibit a slight systematic tendency for the uptake coefficient on NaCl solution to be greater than on pure water, indicating that the uptake of N2O5 by these aqueous media might be reaction-controlled. This assumption leads to a lower limit of 800 mol L(-1) atm(-1) s(-1/2) for the product Hk(1/2) from a simple steady state model (where H is the Henry’s law constant for N2O5 and k is the first-order hydrolysis rate constant).