A method for measurement of size distributions of unknown aerosols is described. 220Rn decay products were attached to particles and the radioactive aerosol was deposited in a spiral centrifuge. By measuring the distribution of activity along the brass foil of the spiral centrifuge, the relative size distribution of the aerosol was determined. After calibration with nearly monodisperse latex spheres, the size distributions of cigarette smoke, the particulate phase of motor car exhaust gas and the atmospheric aerosol were determined.