The design and development of a coaxial capacitance transducer for on-line measurement of particulates in air is described. The system's performance is evaluated in response to changes in a number of operating parameters including relative humidity (8-78%), temperature (20-100 o C) and flow velocity (6.5-15 m/s). Important particulate characteristics investigated include electrical properties, density, mean size and shape. It is found that the effective dielectric constant, e f f , for all solids-gas dispersions tested is directly proportional to the solids concentration. However, in contrast to that for conducting powders, e f f for dispersions of insulating powders is found to be also dependent on the respective dielectric constant of the constituent solid particles, volumetric ratio as well as mean size. A 'temperature capacitor coefficient' is determined to account for the effect of temperature on the system's signal. Typical system particulate volumetric concentration resolution in the range 0-0.004% v/v is +/-2x10 - 4 %.