Indium oxide thin films were deposited by thermal evaporation and tin oxide samples by spray hydrolysis technique. Glass slides were used as substrate. For indium oxide (IO) samples the impurity layer was sandwiched between two IO layers in half part of the substrate; while, the other part consisted of two layers of IO films only. For diffusion of impurity into IO, the substrates were then heat-treated in air (at 500°C for 30 min.). Three bands of silver electrodes were evaporated onto the middle and end sides of each slide. This way, the sensitivity of the doped and undoped IO sides could be compared with one another. Effect of Sb, Se, Ni, Cr, and Cd impurities on sensitivity of IO samples to 2000 ppm hydrogen gas (at the atmospheric pressure) was measured. The results showed that Ni and Se impurities produced highest sensitivities (58.9% and 57.8% respectively). Tin oxide (TO) films were prepared by spraying a tin chloride solution onto heated (480°C) substrates. For doped TO samples, a fixed amount of the impurity chloride was added to the spray solution (in case of F doping, ammonium fluoride was used). Effect of doping with Sb, Ni, Cd, Ag, Zn, Cu, In, and F impurities on sensitivity of TO gas sensors to 2000 ppm hydrogen gas was studied. For TO samples, In and Ag impurities produced the highest sensitivities (20.5% and 20.3% respectively). The evaporated IO samples were generally more sensitive to hydrogen gas, while the sprayed TO samples had higher longterm stability.

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