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In situ atomic force microscopy (AFM) and scanning tunnelling microscopy (STM) studies were made for RF cathode sputtered metal nanolayers on tin-dioxide gas sensor surfaces during the gas sensing process. Our earlier AFM and resistivity measurements revealed the agglomeration of layers during heating. The present in situ AFM and, especially, STM results show a change in the picture quality after hydrogen adsorption, but no changes were found in the grain size during the gas sensing process. As the surface morphology seems to stay unchanged, it may be concluded that the gas response at the exposure is based only on electronic and atomic processes, i.e., changes in work function, surface and interface potential barriers and charge carrier concentration.