Sampling of sea-salt particles was carried out over the western Pacific Ocean. The collected particles were studied individually by X-ray spectrometry. Sea-salt particles with radii r≥0.5 &mgr;m show a considerable range in their weight ratios of S/Na and Ca/Na, while Cl/Na ratios remain constant at the seawater value. Moreover, there is a tight correlation between S/Na and Ca/Na. This property is attributable to fractionary recrystallization within evaporating drops. Particles with r<0.5 &mgr;m exhibit a similar trend. However, these particles tend to show smaller Cl/Na ratios and larger S/Na ratios than those with r≥0.5 &mgr;m. Chemical reaction between sea salt and acidic material, where Cl- is replaced by (SO4)2- and (NO3)-, is the most plausible explanation. ¿ American Geophysical Union 1993