Display omitted •ZnO-CuO composite thin films successfully synthesized on quartz substrate.•The 7mole% CuO content film shows selective response (65%) towards CO gas.•Beyond 8.0mol% CuO content film shows ‘n’ to ‘p’-type CO sensing feature.•The preferential CO adsorption leads to selective response.•The point defects (VO¨, Zni¨) plays crucial role to get selective CO response. In the present work we have demonstrated CuO-ZnO composite thin film with optimized CuO content selectively sense carbon monoxide gas. The ‘n’-type gas sensing characteristics of these composite films are changed to ‘p’-type beyond 8.0mol% CuO contents. Through the analyses of photoluminescence spectra in conjunction with XPS we have demonstrated that up to 33.0mol% CuO contents the gas sensing characteristics of the composite films are controlled by the type of point defects (oxygen vacancies, zinc interstitials, and copper vacancies) in CuO and ZnO grains. We have hypothesized that these point defects help to chemi-absorb oxygen and CO preferentially on ZnO and CuO grains respectively to yield selective carbon monoxide sensing of ZnO-CuO composite film with 7.0mol% CuO contents. Beyond 33.0mol% CuO contents the effect of point defects diminishes and the gas sensing characteristic are grossly controlled by the nature of the CuO grains covering ZnO grains in the composite films. Our work provides comprehensive insight towards the understanding of the gas sensing characteristics of hetero-composite thin films deposited using a mixed precursor sol.