The CuO/ZnO heterocontacts for carbon monoxide (GO) gas sensors were made using CuO ceramics with different histories, and their gas sensing characteristics were evaluated. The CuO/ZnO heterocontact made by the quenched CuO sintered compact showed enhanced CO and CO2 gas sensitivity, while that made by slowly cooled CuO showed no selective CO gas-sensing characteristics with no CO2 gas sensitivity. The quenched CuO ceramics were thermodynamically stable but the slowly cooled ones were not; therefore the adsorbed CO molecules on the quenched CuO are thought to be stable and have longer lifetimes. The specific reaction between the adsorbed CO on CuO and adsorbed oxygen on ZnO would be promoted by long lifetime adsorbed CO on CuO and that may be the origin of the enhanced CO gas sensitivity of the quenched-CuO/ZnO heterocontact. The concentrations of cation vacancies of the quenched CuO ceramics can be controlled by controlling the firing temperatures. However, when the CuO ceramics for the CuO/ZnO heterocontact were made by firing and quenching, the ability to be selective in CO gas sensing of the heterocontact was not easily controlled by the control of the defect concentrations of the CuO ceramics.