The measurement of the radioactive aerosol size distribution was performed using a low pressure cascade impactor (Berner Impactor, Hauke Inc. ) with 9 stages. One impactor (sited in Vienna) had two additional stages in the Aitken particle range. Thin polyethylene foil was used as collection medium on each impactor stage. Gamma spectrometry with high purity Germanium detectors was made of each foil of collected activity. The integrated activity of each set of impactor foils was in good agreement with filter measurements of the airborne activity of the same time period. Study results are discussed.