The water-soluble fraction of atmospheric aerosol particles is a major property dividing the particle population into cloud condensation nuclei and interstitial particles during cloud formation. Likewise, this property influences cloud microphysics and chemistry as well as direct and indirect aerosol forcing of climate. Up to now this important parameter has been only poorly determined experimentally. Here, we present SoFA, a new method to determine the water-Soluble Fraction of large and giant Aerosol particles in five narrow size bands with geometrical radii of 0.4, 0.6, 0.9, 1.3, 1.8 μm and one band for particles with radius larger than 2.3 μm. First results show three different types of aerosol particles (AP I, II, III). AP I were characterized by 9% water-soluble material, and AP II by 50% and correspond to those found in earlier studies for small particles. The new AP III consists of 88% water-soluble material. In those size bands, where all three particle types could be detected (0.9, 1.3, and 1.8 μm; because of detection limits of SoFA, only two types of particles could be detected in the remaining size bands), about 50% of the total number of analyzed particles belong to AP III, whereas about 25% belong to both AP II and I. These numbers lead to 59% water-soluble material on average. Particles of AP III are assumed to be cloud-processed particles, those of AP I might be of biological origin. The impact of the water-soluble fraction of particles on cloud microphysics is investigated by comparing two cloud situations using an entraining air parcel model with detailed microphysics. The model simulations show an influence on nucleation scavenging, cloud interstitial aerosol, and in particular, drop size population. Most notable is that the same aerosol particle number distribution causes either a precipitating or a non precipitating cloud, depending only upon different water-soluble fractions of the particles. ¿ American Geophysical Union 1996