An impedancemetric thick-film CO sensor using RE₂CuO₄ (RE = La, Sm, Eu, Gd) oxides was examined for monitoring carbon monoxide. The thick-film sensor using Eu₂CuO₄ showed a high response to CO. The response was almost linear to CO concentration between 50 and 600 ppm, and the 90% response time to 300-ppm CO was approximately 90 s, although the CO response of the sensor was affected by NO and NO₂ at low concentration.