[en] For the last few years many French dwellings have been monitored during the course of regional or national surveys to assess radon and radon daughters concentrations. In some houses in Britanny, where a diagnosis of radon sources was performed because of relatively high radon levels, a pilot study was carried out in order to characterise indoor radon daughters. The present paper describes the methodology used to measure the equilibrium factor, unattached fraction and the activity weighted size distribution of radon daughter aerosols, in connection with characteristics of ambient aerosols. Data from five individual houses revealed that the activity size distribution of the attached fraction was, in particular, dependent on the indoor ambient particle spectra. It was also found that the unattached fraction in terms of potential alpha energy concentration (PAEC) ranged from 0.008 to 0.49 with a mean value of 0.086, higher than the reference value generally considered. (author)