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Four carbon monoxide microsensor based on semiconducting oxide ZnO:Ga has been developed. The results and analysis of the characterization in an atmosphere containing carbon monoxide (CO), are presented in this work, dilution CO is 50ppm. ZnO:Ga thin films were deposited at 450°C by the spray pyrolysis technique from a 0.2 M starting solution with a [Ga]/[Zn]= 3 at %. Microsensors with four different dimensions were designed: 20x20μm2, 20x40μm2, 20x60μm2 and 100x100μm2. Ohmic contacts were manufactured by thermal evaporation of aluminum on the top of the films. Both of gas microsensors and AI terminals were patterned by lift off. A surface resistance variation of several orders of magnitude was found in doped-gallium ZnO thin films when these were introduced into a camera with 0ppm, 1ppm, 5ppm, 50ppm and 100pm of CO. Thin films gas microsensors were tested at several temperatures and different CO concentrations. The measurement temperatures employed were 200°C, 250°C and 300°C. The surface of ZnO thin films doped with Ga was also characterized by AFM, showing a regular and uniform morpholgy.