The AC electrical properties of the mixed conducting oxide Li0.9Ni0.5Co0.5O2-delta at 1592 Hz have been monitored during insitu temperature programmed exposure to a sequence of dry and/or humid gaseous atmospheres between ambient and 400 degrees C. TGA data was correlated with the electrical measurements. Dry oxygen had little effect. Humidity resulted in a significant increase in conductance and capacitance in argon but not in oxygen. Changes in conductance and capacitance arise from changes in both surface conductance and surface dipoles during chemisorption and reaction processes. Following n-type behaviour, in both propylene:air and CO:air treatments, the conductance was increased, indicating reduction of the catalyst surface. Very low conversion (<5%) was obtained for propylene (mainly to carbon dioxide), while a higher conversion (maximum of about 97%) was achieved for CO which indicates that Li0.9Ni0.5Co0.5O2-delta could be a possible catalyst to control pollution.