The electron microscope has been used in studies of the nature of methane sulfonic acid (MSA) particles collected from the air of Sakushima Island in Japan, near the Pacific Ocean. Particles were impacted on electron microscope screens for morphological analysis and thin-film chemical tests. MSA particles could be identified on the basis of their distinctive morphology on the collection surface. In contrast to MSA, sulfuric acid has a similar morphology on a carbon film, but a different morphology on a nitron-barium chloride film. It was found that MSA-containing particles were dominant in the coarse-particle fraction under very humid conditions. The examination also suggested that the MSA particles were present as mixed particles. The number concentration of MSA particles varied from 0 cm-3 to 10-1 cm-3. Possible mechanisms for the formation of MSA-containing particles are discussed in light of the field measurements. ¿ American Geophysical Union 1993