A polymer-assisted deposition method was used to prepare nanocomposite structures of cobalt oxide and single-walled carbon nanotubes (SWCNTs) for a gas sensor application. Scanning electron microscopy images show that the deposition morphology of Co 3 O 4 is closely related to the viscosity and metal ion concentration in the precursor solution. X-ray diffraction data combined with Raman spectra show that the composite film has high crystalline quality. The basic sensing behavior was evaluated with NO x gas as an electron-accepting gas and H 2 as an electron-donating gas. The composite shows a response of ∼200% upon exposure to 4% H 2 at room temperature. This value is higher than that observed in SWCNT thin film and Co 3 O 4 film, as well as in other pure and functionalized CNTs. The enhancement may have originated from the combination of the nature of Co 3 O 4 particles that actively react with gases and the formation of an electrical continuum between Co 3 O 4 and SWCNTs.