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Nanocrystalline SnO2 and SnO2:Cu thin ˉlms derived from SnCl2 ? 2H2O precursors have been prepared on glass substrates using sol–gel dip-coating technique. The deposited ˉlm was 300 ? 20nm thick and the ˉlms were annealed in air at 500?C for 1 h. Structural, optical and sensing properties of the ˉlms were studied under di?erent preparation conditions, such as Cu-doping concentration of 2%, 4% and 6 wt.%. X-ray di?raction studies show the polycrystalline nature with tetragonal rutile structure of SnO2 and Cu:SnO2 thin ˉlms. The ˉlms have highly preferred orientation along (110). The crystallite size of the prepared samples reduced with increasing Cu-doping concentrations and the addition of Cu as dopants changed the structural properties of the thin ˉlms. Surface morphology was determined through scanning electron microscopy and atomic force microscopy. Results show that the particle size decreased as doping concentration increased. The ˉlms have moderate optical transmission (up to 82.4% at 800 nm), and the transmittance, absorption coe±cient and energy gap at di?erent Cu-doping concentration were measured and calculated. Results show that Cu-doping decreased the transmittance and energy gap whereas it increased the absorption coe±cient. Two peaks were noted with Cu-doping concentration of 0–6 wt.%; the ˉrst peak was positioned exactly at 320nm ultraviolet emission and the second was positioned at 430– 480 nm. Moreover, emission bands were noticed in the photoluminescence spectra of Cu:SnO2. The electrical properties of SnO2 ˉlms include DC electrical conductivity, showing that the ˉlms have two activation energies, namely, Ea1 and Ea2, which increase as Cu-doping concentration increases. Cudoped nanocrystalline SnO2 gas-sensing material has better sensitivity to CO gas compared with pure SnO2.