The heterogeneous reactive uptake coefficients of N2O5 on metastable and deliquescent aerosols were determined in a large aerosol chamber at different humidities to 0.0018¿0.0004, 0.0032¿0.0006, and 0.023¿0.005 at 48, 62, and 88% relative humidity and 293¿2 K. The uptake coefficients at the lower two relative humidities are ~1 order of magnitude smaller than the uptake coefficients of N2O5 on other aqueous surfaces. This effect is explained by the ionic hydrolysis mechanism of N2O5 and the influence of the significantly enhanced recombination reaction NO2++NO3-⇒N2O5 due to the large molality of NO3- in the metastable aerosols. About 70% of the heterogeneously hydrolyzed N2O5 are found to be released as HNO3 into the gas phase. The results present experimental support for the ionic hydrolysis mechanism of N2O5. The low reaction probabilities lead to lower heterogeneous NOx turnover in specific areas of the atmosphere as assumed so far. ¿ 1998 American Geophysical Union