In general, solid electrolyte based potentiometric CO2 sensors respond well to changes in CO2 concentration, following Nernst equation when dry CO2 is used. Unfortunately, the sensing capability will be fatally disturbed when these sensors are exposed to humid conditions. To overcome this problem, a sensor using a porous BaCO3 film as counter electrode (p-Sensor) was fabricated. For the purpose of comparison, sensor without this porous structure (o-Sensor, i.e. an opened counter electrode) and sensor with a dense BaCO3 film (d-Sensor) also have been fabricated. The electromotive force (EMF) of all sensors exhibited excellence Nernstian behavior with the logarithm of CO2 concentration in the range 300-755 ppm at 400 degrees C under dry condition. However, the EMF values of each sensor tended to shift upward with increase of relative humidity. It was found that the relative humidity dependence of EMF originates from both of auxiliary and counter electrodes. Under the same humid conditions, the order of the EMF deviation of three types of sensors was shown as the following: d-sensor>p-sensor>o-sensor. Nevertheless, only p-sensor still remained the Nernstain behavior even under humid conditions. The electron transfer numbers are in good agreement with theoretical value of n=2. Moreover, the transients were sufficiently sharp, taking less than 1 min for 90% response or recovery. The most important thing is EMF can rapidly recover the original value without any deterioration. The reason for the satisfactory performance of p-sensor under humid condition was suggested to be due to the amount of H2O molecular adsorbed on the porous counter electrode is very close to that of auxiliary electrode. Published by Elsevier B.V.