In2O3-doped SnO2 nanoparticles were prepared using sol-gel technique from 0.1-M solutions of both stannic chloride (SnCl 5H(2)O) and indium nitrate. The doping concentration was varied from 7.718 x 10(-5) to 3.859 x 10(-4) moles. The average particle size, as measured from XRD, SEM, and TEM analyses, varies from 34-130 nm as a result of powder calcination at different temperatures ranging from 300 degrees C-900 degrees C. Thick-film samples with a thickness of similar to 15 mu m, were tested for low concentration (15-1000 ppm) of CO in air ambient. The optimal temperature for CO sensing is found to be 220 degrees C-240 degrees C. A blue shift in the sensing temperature and increase in sensitivity factor (S-f) is observed with increasing doping concentration of indium oxide. Maximum sensitivity factor of similar to 5 is found for the highest doping concentration (3.859 x 10 (-4) moles) at 1000 ppm of CO concentration. The morphological and elemental studies of the film are carried out using SEM, TEM, XRD, an techniques. The results are discussed based on elemental an lyses and available theories.