Semiconductor sensors based on nanocrystalline SnO2, In2O3 and In2O3-SnO2 thin films have been investigated for detecting low concentration (2-20 ppm) of nitrogen dioxide in dry air. In this work the gas-sensitive layers were prepared by modified sol-gel methods making use of no-standard precursors and a suitable surfactant. The samples have been morphologically characterized by SEM. Good gas-sensing responses towards NO2 have been found for all the prepared samples with improved performances for the In2O3-SnO2 based sensor. The performances of the sensors have been discussed according to the surface chemical reactions between the gas phase and the semiconductor.