A nano differential mobility analyzer (DMA) is used to measure both the size and electrical charge distributions of soot particles generated during rich premixed combustion. The size distributions are bimodal. One mode peaks at diameters below the 3 nm lower limit of the nano DMA and falls off nearly exponentially with increasing particle diameter. The intensity of this mode persists with increasing height above the burner suggesting that it represents the continued formation of new particles. The second mode is lognormal in shape. Its intensity decreases and the mean diameter increases with increasing height above the burner due to coagulation and surface growth as the particles rise in the flame. The DMA measurements show that a substantial fraction of the soot particles are electrically charged in the flame, predominantly with a single charge per particle and with essentially equal numbers of positive and negative particles. These charged particles belong solely to the upper mode, whereas the lower mode remains charge neutral, suggesting that ions do not act as soot nuclei. Following soot inception, the fraction of charged particles quickly increases with height above the burner and stabilizes at similar to30% of the upper mode for each polarity. (C) 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.