The effect of thermal treatment and additives on surface chemistry and gas sensor properties of nanocrystalline tin oxide is analyzed. SnO₂–Fe₂O₃ and SnO₂–MoO₃ nanocomposites were prepared by wet chemical synthesis. The composition and nanostructure of the powders have been previously investigated by EDX spectroscopy, XRD, TEM, Raman spectroscopy, BET measurements and Mössbauer spectroscopy. Acidic properties of the samples were characterized by NH₃-TPD method. The raising of treatment temperature for undoped SnO₂ changes the activation energy spectra of ammonia desorption. The concentration of acid centers on the surface of SnO₂ increases with addition of acidic oxide MoO₃ and decreases with addition of basic oxide Fe₂O₃. In the case of NH₃ detection the sensor signal value increases with the density of acid centers for both SnO₂–Fe₂O₃ and SnO₂–MoO₃ series. For ethanol detection the sensor signal mainly depends on the nature (Brønsted or Lewis type) of acid centers.