The size and electrical charge distributions of soot particles generated in rich premixed flames are examined using a nano-differential mobility analyzer (nDMA). The aim is to investigate how these distributions vary with the choice of fuel, diluent, and flame gas velocity. The measured soot size distributions are typically bimodal. The dynamics of the upper size mode is qualitatively independent of the fuel. At increasing heights above the burner this mode increases in diameter and volume fraction, but decreases in number concentration, as expected from surface growth and coagulation. At about 6 mm above the burner a small fraction of the particles, < 10%, acquire a bipolar charge. The charge is associated with the tipper mode, where the fractions of positive and negative particles evolve to a Boltzmann distribution, whereas the lower mode remains electrically neutral. The existence of the lower mode depends sensitively on the choice of fuel, the flame gas velocity, and the diluent. Comparison to model calculations of flame structure reveals that as each of these are varied, the lower mode exhibits an inverse correlation with temperature, hydrogen atom, and pyrene concentrations. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.