A parallel‐plate electrostatic aerosol size classifier has been constructed, analyzed, and tested. The instrument can produce sub‐micron aerosols with high output, good resolution in the size distribution, and of either single or mixed charge polarities. Performance tests were conducted on latex aerosols and on coal dust, and the instrument was used to calibrate a laser particle size analyzer on coal dust. Calculated and measured output sizes agree to about 7%. A size resolution of approximately 13% for monodisperse aerosols and 17% for a polydisperse coal dust aerosol was obtained.