Measurements on the composition of nanometer-sized atmospheric particles are the key to understand which vapors participate in the secondary aerosol formation processes. Knowledge on these processes is crucial in assessing the climatic effects of secondary aerosol formation. We present data of >2 nm particle concentrations and their hygroscopicity measured with the Condensation Particle Counter Battery (CPCB) at a boreal forest site in Hyytiälä, Finland, during spring 2006. This is the first time such results on the hygroscopicity and composition of the freshly formed atmospheric clusters as small as 2 nm have been reported – bringing us closer to online measurements of the compounds participating in atmospheric particle formation. The data reveal that during new particle formation events, the smallest particles activate for growth at clearly smaller sizes in water than in butanol vapor. However, even at 2–4 nm, the particles are less hygroscopic than ammonium sulfate or sulfuric acid, which are often referred to as the most likely compounds present in atmospheric nucleation. This observation points to the presence of water-soluble organics, even at the very first steps on particle formation. The water-affinity of the particles decreases with size, indicating that the vapors that participate in the first steps of the particle formation and growth are more hygroscopic than the vapors contributing to the later stages of the growth. This suggests that the relative role of less hygroscopic organics in atmospheric particle growth increases as a function of particle size.