Nanocrystalline tin dioxide powders have been produced by mechanochemical milling followed by heat treatment and subsequent preparation of SnO2 thick-films. X-Ray powder diffraction was used to investigate the structure of the materials. Crystallite sizes in the range from 15 to 20 nm (determined from XRD line broadening) were obtained. Heat treatment of the milled powder resulted in the formation of tetragonal and orthorhombic SnO2 phases, which was confirmed by XRD analysis. The optimal parameters for the milling time, rotation speed and heat treatment were determined before this work and used in the current work. The gas-sensing properties of tin dioxide can be modified by adding of La2O3 as surface catalyst. La2O3 was added by dipping sintered tin dioxide films into varying concentrations of La(NO3)(3).6H(2)O solution. The response of the prepared thick-films to HA in the concentration range from 0.5 to 20 ppm in air, revealed that at a temperature of 250 degreesC, undoped SnO2 sensors respond to H2S much stronger than La2O3-doped SnO2 sensors do.