Standard aerosol instruments to measure particle size distributions in the ultrafine size range are large and heavy. We built a small electrical diffusion battery, which can be carried in a backpack and thus makes size-resolved short-term personal aerosol exposition measurements possible. The instrument was designed for maximal measurable particle size range and long maintenance intervals. The optimum number of stages for the diffusion battery was found with a Monte-Carlo simulation. To assess the instrument's performance, we compared size distributions measured with the electrical diffusion battery to those obtained with standard aerosol instruments (the scanning mobility particle sizer and the fast mobility particle sizer of TSI Inc.). In general, the readings of the electrical diffusion battery and those of standard instruments agree well (to within 10-20%). (C) 2008 Elsevier Ltd. All rights reserved.