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Active layers for detecting NO₂ have been obtained by adding a low amount of oxygen-functionalized MWCNTs to three different types of metal oxides (SnO₂, WO₃ or TiO₂). We show that the responsiveness towards NO₂ of micro-sensors based on some of the resulting metal oxide/MWCNT hybrid films is considerably improved. In particular, the sensors based on hybrid SnO₂/MWCNTs films present excellent sensitivity towards NO₂ when operated at room temperature. Additionally, the sensors show a total recovery of their baseline resistance and reasonable response and recovery times. A mechanism of response based on the development of two depletion layers, one at the surface of metal oxide grains and another at the interface of the n-metal oxide/p-MWCNT hetero-structure, is postulated as responsible for the improvement observed. Our results suggest that there should be an optimum amount of carbon …