The gas sensing characteristics of CuO/ZnO heterocontacts are investigated. CuO/ZnO heterocontact, with Na2CO3 added, showed selective CO gas sensing properties, while pure CuO/ZnO heterocontact showed higher sensitivity to H-2 than to CO. It is suggested that the presence of a second phase containing Na+ in CuO ceramic is essential for selective CO gas sensing by a heterocontact.