A particle growth system (PGS) of original design is presented here, which, when combined with an opticle particle counter has the same function as a condensation nucleus counter (CNC). After an experimental study validated by a numerical model of transfers in a continuous-flow condenser, a relationship of unambiguous dependence is established between the droplet sizes and the size of the primary particles when the latter are between 4 and 20 nm. Furthermore, when aerosol sample is made up of an ultrafine fraction and a fine fraction (about 50 nm), the size distribution of the droplets obtained from the PGS is shown to be bimodal too. Finally, the activation efficiency of our PGS is found to be of the order of 100% for particles as small as 4 nm.