Nanocrystalline SnO2 and SnO2:Cu thin ¯lms derived from SnCl2 � 2H2O precursors have been prepared on glass substrates using sol–gel dip-coating technique. The deposited ¯lm was 300 � 20nm thick and the ¯lms were annealed in air at 500�C for 1 h. Structural, optical and sensing properties of the ¯lms 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 ¯lms. The ¯lms 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 ¯lms. Surface morphology was determined through scanning electron microscopy and atomic force microscopy. Results show that the particle size decreased as doping concentration increased. The ¯lms 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 ¯rst 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 ¯lms include DC electrical conductivity, showing that the ¯lms 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.