A large fraction of atmospheric particles are composed of hygroscopic salts that are mixed with variety of organic molecules, of which surfactants represent an important and interesting class. We present an experimental study of the effect of mixing a soluble surfactant with two hygroscopic salts on particle density, shape and water uptake. We show that the measured densities as a function of particle compositions provide evidence that the surfactant retains water even at very low RH and that its density changes with concentration and is significantly different from that of the anhydrous crystal. When this behavior is taken into account to calculate the effect of surfactant on particle hygroscopicity, the water uptake data can be quantitatively understood. The vacuum aerodynamic size distributions indicate that drying particles with intermediate surfactant concentrations produces a range of particle types, whose properties can be altered by heating. (C) 2007 Elsevier Ltd. All rights reserved.