A new potentiometric soot sensor for in situ application based on a porous zirconia solid electrolyte has been developed. Its mode of operation has been evaluated in a test facility with test gas of various soot concentrations. Stationary soot concentrations in the test gas ranging from 0 to 90 m&m3 could be produced with the help of an aerosol generator. The sensor signal was found to be in the range of 0 to 60 mV with higher sensitivity at low soot concentrations (<30 mg/m3) and approaching saturation at concentrations >70 mg/m3. Sensitivity and response time were found to be inversely temperature dependent: higher temperature results in a faster response, but a lower sensitivity. Sensitivity in the low concentration range is increased when the gas atmosphere contains less O2 than air and the saturation of the sensor signal appears already at some 30 mg/m' in this case. Noncombustible particulates do not contribute to the sensor signal. The working principle of the sensor can be understood by a first-order model assuming an equilibrium between adsorption and burn-off of soot particulates at the working electrode, and regarding the sensor as a network of distributed e.m.f. sources.