Differential heats and isotherms of ethanol adsorption in a zeolite H3,25ZSM-5 have been measured by Tian-Calvet-type microcalorimeter and volumetric system at 303 K. Based on the data obtained, the mechanism of ethanol adsorption and (C2H5O)n/Н+ complexes formation in the zeolitic matrix of H3,25ZSM5 is revealed. The adsorption isotherms are quantitatively reproduced by VOM theory equations.