Most of the airborne radon-222 daughters are attached to aerosol particles. An experimental set-up for measuring the radon daughter activity as a function of the aerosol particle size is described. An electrostatic classifier selects particles of a certain size range from the indoor or natural aerosol. An arrangement of a filter and an alpha detector registers the radioactivity of the particles. Simultaneously the aerosol concentration is measured by an aerosol electrometer which is upgraded for two particle concentrations. | Most of the airborne radon-222 daughters are attached to aerosol particles. An experimental set-up for measuring the radon daughter activity as a function of the aerosol particle size is described. An electrostatic classifier selects particles of a certain size range from the indoor or natural aerosol. An arrangement of a filter and an alpha detector registers the radioactivity of the particles. Simultaneously the aerosol concentration is measured by an aerosol electrometer which is upgraded for low particle concentrations.