Limiting-current-type oxygen sensors using BaCe0.8Gd0.2O3-alpha (BCG) ceramics as electrolytes were constructed on a trial basis and their sensing properties investigated in order to develop a new oxygen sensor to replace the zirconia type. BCG ceramics exhibited high conductivity in the temperature range 200-1000 degrees C, and it was verified that the oxide ion could be a conductive carrier in BCG in oxygen at low temperatures (300 degrees C). The oxygen sensors using BCGs worked at 300 degrees C, and their output currents linearly increased with an increase in oxygen concentration in the range 1-22%. They could respond within 30 s between 1 and 21%, and humidity only slightly affected sensing performance. BCG seems to be a promising electrolyte material for an oxygen sensor operating at low temperatures (300 degrees C).