The design and development of a co-axial 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 oC) 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, eeff for all solids-gas dispersions tested is directly proportional to the solids concentration. However, in contrast to that for conducting powders, eeff 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 2 x 10-4 %.