The influence on gas-phase catalytic glycerol dehydration of crystal size (S: small, or L: large crystals), acidity, and synthesis procedure for isomorphous incorporation of gallium (Ga-S; Ga-L) or aluminum (Al-S; Al-L) in MFI zeolites was studied. The main product observed was acrolein, with the undesirable parallel formation of deactivating coke molecules such as polyglycols and polyaromatics. The Ga-S zeolite showed the best performance in this reaction, as it provided a combination of adequate accessibility to the microporous system and weak Bronsted acid sites. The chemical and structural properties of the fresh MFI zeolites were studied by X-ray diffraction, nitrogen sorption measurements, scanning electron microscopy, temperature-programmed desorption of NH3, X-ray photoelectron spectroscopy, and Al-27 and Si-29 MAS-NMR. Solid-state C-13 MAS-NMR and thermogravimetric analyses of the spent MFI zeolites confirmed the differences in the nature and amounts of the carbonaceous deposits formed. The polyglycols were preferentially formed on the external surface of the zeolite crystals, as expected due to the greater exposed area. On the other hand, the polyaromatic compounds formed were more abundant inside the micropores of the MFI zeolites, especially those composed of larger crystals and with a greater number of strong Bronsted acid sites. (C) 2015 Elsevier B.V. All rights reserved.