Industrialization and urbanization are the main causes of increased gas emissions in recent years. The emissions of gases such as NO2, H2S, CO and CO2 from various industries are often harmful to the well-being of living organisms and the earth’s atmosphere. In order to preserve lives and the earth’s atmosphere, the development of gas sensors are of high importance. Zinc oxide (ZnO) nanostructures were synthesised using hydrothermal microwave oven digestion method. Zinc sulphate (ZnSO4) and sodium hydroxide (NaOH) were used as metal precursor and reducing agent, respectively. In an attempt to improve the surface area and the electrical conductivity of the ZnO, carbon nanotubes (CNTs) were synthesised using chemical vapour deposition and were used to make the CNTs/ZnO composite. Characterisation techniques such as Brunauer–Emmett–Teller (BET), Scanning Electron Microscope (SEM), X-Ray diffractometer (XRD) and Transmission Electron Microscope (TEM) were used. The NaOH concentration of 0.3 M yielded ZnO nanosheets with the surface area of 102 m2/g. The prepared CNTs/ZnO composite had a surface area of 153 m2/g. Gas sensing analysis was conducted using carbon monoxide (CO) as an analyte gas at 250°C. The sensitivity and response time of the ZnO were calculated to be 9.7% and 114 seconds, respectively, at a CO concentration of 200 ppm. The CNTs/ZnO composite showed the highest sensitivity of 9.9% and fastest response time of 49 seconds when exposed to 200 ppm of CO gas.