Hebb-Wagner (HW) studies in 10(-3)-1 atm of oxygen, using an ion blocking electrode, show that Ce0.8SM0.2O1.9 (CSO) is predominantly oxide ion conducting in the temperature range 500-600degreesC. The bulk ionic conductivity data are in agreement with those reported in the literature. The diffusion coefficient of the electrons is found to be similar to10(-6) cm(2)/s at 500-600degreesC. This value is of same order of magnitude as the diffusivity of oxygen reported for Gd-doped CeO2 (CGO) (similar to10(-6)-10(-5) cm(2)/s at 500-970degreesC). The excess electronic conductivity is found to be about three orders of magnitude lower than the total conductivity with an activation energy of 1.2-1.6 eV in the oxygen partial range 10(-3)-1 atm, which is comparable to those of other rare earth doped cerates. Application of CSO in amperometric oxygen sensors at 550degreesC shows that the limiting current varies according to Fick's law in the oxygen concentration range between 100 and 500 ppm. It is found that at higher oxygen partial pressures than 500 ppm the measured current is lower than the calculated one. (C) 2003 Elsevier Ltd. All rights reserved.