Pure nanocrystalline SnO2 ¯lms were grown on a clean glass substrate by using sol–gel dip coating and chemical bath deposition (CBD) techniques for gas sensor applications. The ¯lms were annealed in air at 300�C, 400�C, and 500�C for 60 min. The deposited ¯lms with a thickness of approximately 300 � 20 nm were analyzed through X-ray di®raction, scanning electron microscopy (SEM), atomic force microscopy (AFM), and optical absorption spectroscopy. Results revealed that the ¯lms produced by dip coating exhibited a tetragonal rutile structure and those produced by CBD showed a tetragonal rutile and orthorhombic structure. The crystalline sizes of the ¯lms produced by dip coating annealed at 300�C, 400�C, and 500�C were 8, 14, and 22.34 nm and those for CBD ¯lms at these temperatures were 10, 15, and 22 nm, respectively. AFM and SEM results indicated that the average grain size increased as annealing temperature increased. The transmittance and absorbance spectra were then recorded at wavelengths ranging from 300nm to 1000 nm. The ¯lms produced by both the methods yielded high transmission at visible regions. The optical band gap energy of dipcoated ¯lms also increased as annealing temperature increased. In particular, their optical band gap energies were 3.5, 3.75, and 3.87 eV at 300�C, 400�C, and 500�C, respectively. By comparison, the energy band gap of CBD-prepared ¯lms decreased as annealing temperature increased, and their corresponding band gaps were 3.95, 3.85, and 3.8 eV at the speci¯ed annealing temperatures. The ¯lms were further investigated in terms of their sensing abilities for carbon monoxide (CO) gas at 50 ppm by measuring their sensitivity to this gas at di®erent times and temperatures. Our results demonstrated that dip-coated and CBD-prepared ¯lms were highly sensitive to CO at 200�C and 250�C, respectively.