AbstractThe use of electron microscopy for observation and measurement of solid particles in air pollution is described, the thermal precipitator being used as sampling instrument. By using heat‐stable membranes, it is possible to distinguish combustible from non‐combustible matter, and other chemical tests can be used to distinguish different materials.The shape of the particles in air pollution is mainly spherical; the size varies from 0.01 to 1 μ, although occasional larger particles are found. Many of the spheres are aggregated and resemble carbon black.Examples are given of quantitative evaluation in terms of weight per unit volume for different size classes. The few results obtained so far show reasonable agreement with previous mass estimates based on smoke‐filter sampling. The future developments in the use of the electron microscope for studying air pollution are briefly dis