IN VIEW OF THE INCREASINGLY STRICT LEGAL LIMITS FOR POLLUTANT GAS EMISSIONS, THERE IS A GREAT INTEREST IN DEVELOPING HIGH PERFORMANCE GAS SENSORS FOR APPLICATIONS SUCH AS CONTROLLING AIR POLLUTION AND EXHAUST GASES [1]. IN THIS WAY, SEMICONDUCTOR GAS SENSORS, DUE TO THEIR MANY PARTICULAR ADVANTAGES OVER TRADITIONAL CHEMICAL ANALYSIS METHODS, SUCH AS HIGH RESPONSE, LOW COST AND PORTABILITY, ARE WIDELY USED FOR THE DETECTION OF TOXIC OR FOUL-SMELLING GASES, CHEMICAL PROCESS CONTROL AND MONITORING OF AIR POLLUTION IN THE ENVIRONMENT [2]. CARBON MONOXIDE IS ONE OF THE MOST DANGEROUS GASES IN AIR POLLUTION AND HUMAN LIFE. IT IS HIGHLY TOXIC AND EXTREMELY DANGEROUS. AMONG USEFUL SEMICONDUCTING METAL OXIDES AS SENSING MATERIALS, ZNO HAS BEEN EXTENSIVELY INVESTIGATED FOR DETECTION OF TOXIC AND INFLAMMABLE GASES SUCH AS CO [3]. PURE METAL OXIDE SEMICONDUCTORS HAVE A RELATIVELY POOR GAS SENSING PROPERTY, AND IT IS REQUIRED TO DOPE WITH OTHER ELEMENTS OR OXIDES TO IMPROVE THE PERFORMANCE [4]. DOPING IS AN EFFECTIVE WAY TO TAILOR THE GAS SENSING PROPERTY OF ZNO GAS SENSORS. TWO PARAMETERS DOMINANTLY CHANGED BY THE DOPANTS: THE RESPONSE (OR SENSITIVITY) AND/OR THE TEMPERATURE WHERE THE GAS SENSING MAXIMUM IS GAINED [5]. PURE ZNO, AL AND CU DOPED ZNO thin films AT VARIOUS DOPING CONCENTRATIONS (2, 5, 10 MOL %) WERE PREPARED BY THE SOL–GEL METHOD. THE CO SENSING PROPERTIES OF THE thin films WERE TESTED AT DIFFERENT OPERATING TEMPERATURES AND VARIOUS CONCENTRATIONS OF CO GAS. ALSO EFFECTS THE CONCENTRATION AND TYPE OF DOPANTS ON THE RESPONSE OF GAS SENSORS WERE INVESTIGATED.