As part of the LBA/CLAIRE-98 project (LBA = Large Scale Biosphere-Atmosphere Experiment in Amazonia; CLAIRE = Cooperative LBA Regional Experiment), an extensive aerosol characterisation study was performed. The field work for the study took place in the Amazon Basin from 23 March to 15 April 1998. The collected aerosol samples were analyzed by conventional and thin-window electron probe X-ray microanalysis (EPMA) combined with principal component analysis (PCA). An intensive transport of soil dust particles from the Sahara was observed at the end of March and beginning of April. The absolute number concentration of fine soil dust particles (0.30 µm < particle diameter < 2 µm) reached a value of 3.5 million per m3. Changes in the total number particles and reactions of the Sahara dust, i.e., agglomeration with sea salt above the Atlantic Ocean and with local particulate matter were observed. Particle number concentrations were higher in the fine size fraction, with soil dust dominating in the first part of the campaign. A significant contribution of natural biogenic particles was observed in the last two weeks of the campaign but only in the coarse size fraction. The organic part of the aerosol particles and their agglomeration with other types of particles was studied. The chemical transformation of airborne marine particles in the abundant presence of particles originating from biomass burning and biogenic emissions was observed as well, e.g. N-containing species such as sodium nitrate particles classified as aged sea salt. Carbon was present in almost all particle types.