The chemiresistive gas sensing characteristics of Co₃O₄ thin films – prepared by electron-beam deposition of Co films on (0 0 0 1) Al₂O₃ substrate followed by oxygen annealing at three different temperatures (500^oC, 650^oC and 800^oC) – have been investigated for a host of gases (C₂H₅OH, CO, NO, Cl₂, NH₃ and H₂S). The results of atomic force microscopy and X-ray diffraction revealed that the grains of these films are composed of nano-crystallites (size: 24–56 nm). X-ray photoelectron spectroscopy data revealed that the content of adsorbed oxygen decreases with increasing annealing temperature. For H2S gas in the concentration range 1–100 ppm, films annealed at 650^oC exhibited high selectivity, low base line drift and highest response, which are attributed to small crystallite size, improved crystallinity, better grain-to-grain connectivity and higher adsorbed oxygen content. The H₂S sensing mechanism has been explained on the basis of the depletion of chemisorbed oxygen at the film surface.