Although fluidized bed reactors at elevated temperatures (up to 1000 degrees C) have a wide field of application, there is still a lack of measurement techniques for the key entities : local solids volume concentration and velocity. In this paper, a measurement technique which relies on the measurement of the local dielectricity is described. Through the application of a guard circuit and specially designed probes, an extremely high resolution and stability has been achieved. The system has been successfully operated under high-temperature conditions in a circulating fluidized bed where the local time-averaged solids volume concentration was as low as 0.1%. Measurements of local time-averaged particle velocities and solids volume concentrations in laboratory and industrial fluidized bed combustors demonstrate the wide applicability of the measurement system.