The aim of this work was to investigate the influence of relative humidity of air on the catalytic activity of Ag3PW12O40 and Ag3PMo12O40 salts in the vapor-phase dehydration of ethanol. The reaction was performed in air of relative humidity of 2% or 9% and in the temperature range of 373-493 K. The catalytic activity of Ag3PW12O40 salt was much higher than the one obtained for Ag3PMo12O40 salt and strongly dependent on relative humidity of air. The XRD measurements showed that the structure of Ag3PW12O40 salt loses the water hydrating silver in the range of 473-573 K whereas Ag3PMo12O40 salt in 373-473 K. The FT-IR spectra demonstrated the stability of Keggin structure of Ag3PW12O40 salt after reaction, independently of relative humidity of air. However, the XPS spectroscopy exhibited the changes in surface composition of this salt after reaction in air of 2% of relative humidity whereas the SEM imaging displayed the formation of silver nanostructures on its surface. The Ag3PW12O40 salt remained stable after reaction in air of 9% of relative humidity. In case of Ag3PMo12O40 salt analyzed before and after ethanol dehydration, a very limited difference in surface composition was found by XPS that was supported by SEM imaging. (C) 2011 Elsevier B.V. All rights reserved.