The hygroscopic growth properties of ultrafine NaCl particles between 8 and 50 nm in mobility diameter have been studied using an ultrafine tandem differential mobility analyzer. The data include determination of hygroscopic growth curves, deliquescence behavior, and hysteresis. The measured growth factors are clearly smaller for the nanometer-sized particles compared with particles in the size range of 0.1 μm and larger. While this behavior of the ultrafine particles can be qualitatively predicted by accounting for the Kelvin effect, quantitative differences to the experimental data remain even after the differential mobility analyzer data have been corrected to account for the cubic shape of the particles. We conclude that the differences are related to size-dependent density and/or shape/structure of nanometer-sized NaCl crystals. The deliquescence relative humidities obtained in this work for nanometer-sized particles also show clear differences from the literature value obtained for large particles. The quantitative values of hygroscopic growth factors and deliquescence relative humidities for small particles have not been available previously, and our data are important in studies of the formation and properties of atmospheric aerosols. ¿ 2001 American Geophysical Union