The electrical aerosol analyzer has been studied using monodisperse aerosols. The aerosols were generated by the electrostatic classification principle described by Liu and Pui (1974a) and by the photo-chemical conversion of gaseous SO2 into sulfuric acid droplets. The measuring range of the instrument was found to be from 0·006 to 1·0 μm in particle dia. and the sensitivity was found to vary from 0·01 to 400 pA/(106 particles/cm3) over the range of the instrument. Data are presented showing the performance of the device as a function of particle size and as a function of the operating conditions of the diffusion charger.