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Individual aerosol particles were collected in the middle troposphere at 5–6 km altitude in the PACE-III (Pacific Atmospheric Chemistry Experiment) campaign, conducted between Melbourne in Australia (38°S) and Sendai in Japan (38°N) in July 1995. Particles collected on electron microscope grids were subsequently examined using a transmission electron microscope equipped with an energy-dispersive X-ray (EDX) analyzer. Based on the EDX analysis and morphological identification, particles were classified into seven categories, including sulfuric acid, sulfate, sea-salt and mineral particles. Within the radius range of 0.1–0.5 μm, S-rich particles (sulfuric acid and sulfate) were dominant (> 60%) in the observation regions. These particles are considered an important background aerosol component. The latitudinal distributions of S-rich particles containing potassium (K) and minerals were studied for all S-rich particles of 0.1–0.5 μm. The percentages of K-containing S-rich particles were > 20% north of 35°N and south of 15 S. S-rich particles with K were likely to have originated from biomass or biofuel burning on the continent and were transported through the middle troposphere. In addition, S-rich particles with minerals were often present in relatively high percentages (> 40%) in the northern mid-latitudes. The present study suggests that 30–70% of S-rich particles were influenced by material of surface origin north of approximately 30°N and south of 25°S, where westerly winds were dominant. The results obtained in this campaign were compared with those in the previous PACE-I campaign (January 1994). The comparison suggested that seasonal shifts of the locations of the ITCZ and wind belts play an important role in the formation of the seasonal difference in the spatial distributions of concentration and mixing properties of S-rich particles in the middle troposphere over the western Pacific.