The deposition efficiency of a point-to-plate electrostatic precipitator onto a target substrate was studied in this work. The efficiency when measured in the traditional way, by the loss of particles from the exiting air, was like in many other such devices, typically high (80-90%). However, the actual useful deposition onto the target substrate was never more than 50% and sometime as low as a few percent only. Therefore, special caution should be taken in design and testing of electrostatic precipitators for quantitative sampling of aerosols. (C) 2002 Elsevier Science Ltd. All rights reserved.