The attachment rate of radon decay products to aerosol particles has been measured using a direct reading instrument based on diffusion charging of the particles. The measurements were made continuously over one week periods in 14 Finnish dwellings. A wide range of values was observed ranging from approximately 5 to more than 300 h−1. The high values were observed to be of short duration and caused mainly by occupant activities. The frequency distribution of attachment rate values was found to be approximately log-normal with a median value of 18 h−1.