Aerosol samples were collected on a cruise of the R/V Conrad in the Pacific Ocean. The cruise track was divided into two legs, the first one on the Peru/Ecuador shelf, the second through the equatorial and tropical Pacific to Hawaii. Sampling took place by means of two 1 l/min, ten-stage Battelle-type cascade impactors positioned on the foremast of the ship at about 20 m above the sea surface. Strict precautions were taken to avoid contamination of the samples by the ship itself. The impactor slides and back-up filters were analyzed for 25 elements by PIXE using a 2.4 MeV proton beam, produced by a compact cyclotron. In the samples, collected on leg 1, a significant anthropogenic component could be detected. Excess fine sulfur, excess fine potassium, and V, Ni, Cu and Zn in < 2 μ m particles were as high as 600, 10, 0.8, 0.2, 3 and 4 ng/m 3, respectively. On the other hand, most of the samples, collected on leg 2, were representative of clean marine air which was little influenced by continentally derived aerosols. The sulfur size distribution showed a very pronounced submicrometer mode of about 100 ng/m 3, suggesting a nearby sulfur source of marine origin. After crossing the intertropical convergence zone from south to north, concentrations of Fe and other crustal elements increased significantly. The size distributions and interelement ratios indicated that the elevated concentrations of these elements were due to long-range transport of mineral dust.