In order to determine the resistivity of an electrostatically precipitated particulate layer of high resistivity material, a new technique is described using a conducting potential probe. The space‐charge potentials are first determined by the probe in a small parallel plate precipitator with wire‐to‐plate spacing of 75 mm at several distances from the collector plate, and a short extrapolation gives the potential at the dust surface. Using this potential and the associated measurements of collected layer thickness and current density, the operational resistivity &rgr;fof such a layer is determined while subject to the corona currents and high electric fields of normal electrostatic precipitator operation. The theory is given for the conducting potential probe in a unidirectional ionized field of a single wire to parallel plate unit. An example of the field dependence of the operational resistivity of a fly ash is given.