The hygroscopic and cloud condensation nuclei (CCN) properties of submicrometer atmospheric aerosol particles were investigated using an Ultrafine Tandem Differential Mobility Analyzer (UF‐TDMA) and a CCN counter at the Mace Head Monitoring Station on the west coast of Ireland during the New Particle Formation and Fate in the Coastal Environment (PARFORCE) field campaign in September 1998 and June 1999. These measurements give indirect in situ information on the composition and state of mixing of the aerosol particles. The UF‐TDMA was used for monitoring of hygroscopic diameter growth factors of aerosol particles with dry mobility diameters 8–20 nm when taken from dry state to a controlled humid environment (RH 90%). The CCN counter was used to study the activation of aerosol particles when exposed to supersaturated conditions (dry diameters of 15–150 nm). It was seen that in clean marine air masses during the observed particle formation events, the newly formed nucleation mode particles (8 and 10 nm) most often had low growth factors (between 1.0 and 1.1) resulting from low solubility. This indicates that the nucleation mode particles consist of nonsoluble or weakly soluble species, possible accompanied by a small soluble part; however, when nucleation mode particles were observed outside the event periods, the growth factors were higher (about 1.3–1.4). In contrast, the 20 nm particles usually clearly belonged to the Aitken mode (based on number size distribution measurements) and had hygroscopic properties similar to some common salts (growth factors 1.4–1.5).