Concentrations of the major soluble ions, non-sea-salt (nss) sulfate, nitrate, chloride, and ammonium have been measured in aerosol samples collected at various locations and altitudes over the North Atlantic Ocean. Concentrations of nss sulfate, nitrate, and ammonium were found to be fairly uniform within the maritime boundary layer in an air mass aged in the Azores anticyclone; average values obtained were 0.36¿0.11, 0.18¿0.03, and 0.16¿0.06 μg m-3, respectively. The mean cloud condensation nuclei (CCN) concentration at 1% supersaturation was 368 cm-3, corresponding to air containing the major ion concentrations above. Incursion of continental air with a travel time of about 3 days over the sea led to an increase of around fivefold in nss sulfate, nitrate, and ammonium concentrations, while CCN concentrations increased less than threefold to 1076 cm-3. Two air samples collected during the clean air period above the marine boundary layer showed much reduced concentrations of soluble ions, but a mean CCN concentration of 358 cm-3. Regression analysis of all air sample data indicates that the nss sulfate and nitrate data are correlated to the observed CCN number density. While these ions are clearly making a significant contribution to CCN activity, the degree of correlation indicates that other chemical species also play a role as CCN. ¿ American Geophysical Union 1996