Number size distributions of air ions and aerosol particles were measured at three sites in the Baltic Sea region in spring 2004. One site was on the island of Ut6 in the Baltic Sea and the two other sites, Hyyti5l5 and Tahkuse, had a more continental location not far away from the coast of the Baltic Sea. The concentrations of cluster ions (air ions with a diameter < 1.6 nm) were about three times smaller at the Ut6 island as compared with those at the Hyytidld mainland site in Finland, although the particle concentrations (i.e. ion sink) were at about the same level at these two sites. Generally, the Ut6 island had the lowest air-ion concentration probably due to weaker sources for ions than the other two more continentally-located stations. Intermediate ion concentrations (1.6-7 nm diameter) were generally low, even though they reached several hundreds cm(-1) during nucleation episodes. Charging probabilities of < 8 nm particles were found to be close to the steady state in all the three sites, suggesting that ion-induced nucleation was not playing a major role during this time period.