Si/Zr mesoporous MCM-41 catalysts were investigated in the vapour phase synthesis of 7-ethylindole (7-ETI) starting from 2-ethylaniline (2-ETAN) and ethylene glycol (EG). The catalysts were prepared from the corresponding alkoxides in the presence of a cationic surfactant, which acts as a structure-directing agent. After removal of the surfactant, the resulting mesoporous solids were characterised by X-ray diffraction, N-2 adsorption-desorption, temperature programmed desorption (TPD) of NH3 and pyridine adsorption coupled with FTIR spectroscopy. As the amount of ZrO2 increased in the catalysts, the BET surface area decreased, while an enhancement of the acidity took place. The activation of EG to 2-hydroxyacetaldehyde and its attack on 2-ETAN, leading to the formation of 7-ETI, was favoured by increasing the amount of ZrO2. The maximum yield for the catalyst was obtained with ca. 50 wt.% of ZrO2, which promoted the presence of Lewis and Bronsted acid sites of suitable strength and concentration, although mesoporous catalysts exhibited very good regenerability.