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The effect of chemical oxidation on the gas sensing properties of multi-walled carbon nanotubes (MWNT) is presented in this paper. The MWNT was chemically oxidized in a mixture of concentrated sulfuric and nitric acids. The oxidation time was varied from 4 to 12 hours. X-ray photoelectron spectroscopy (XPS) was used to study effect on the chemical composition of the nanotube surface. The MWNT was deposited onto gas sensor substrate by drop coating method. The gas sensing properties of samples with different oxidation time for NO2 and NH3 were investigated at room temperature. Comparing with the as-obtained MWNT, the gas sensitivity of functionalized MWNT was greatly increased for both NO2 and NH3.