Measurements have been performed for the determination of the size distribution of a coagulating ultrafine aerosol over a time interval of up to about 30 min. The aerosol was contained in a balloon with an initial volume of 60 l subject to a temperature inversion for the purpose of quenching the free convection and thereby diminishing the aerosol loss to the balloon wall. The aerosol size distribution was measured with the TSI electrostatic aerosol classifier hooked up to a TSI aerosol electrometer. The initial aerosol had an average diameter of about 12 nm. Measurements were taken by computer at a rate of 1 measurement cycle every 3 s; 1 cycle consists of a measurement of time, and burst measurements of electrometer current, classifier rod voltage, 3 flow rates, and 5 temperatures, followed by the calculation of averages and standard deviations, and storage of the results in a data string. The TSI instruments have been modified to permit the automatic computer reading of the parameters mentioned above. A multiplexer has been built to allow the multiplet data to be measured by a single system voltmeter. Channel switching in the multiplexer can be done either automatically by using the ''delay''signal emitted by the system voltmeter every time it makes a reading or by software control through the 16-bit parallel interface of the computer.