ZSM-5 zeolite, when doped with fluorine species in relatively low concentration using ammonium fluoride as precursor, and subsequently activated stepwise at high temperature, shows an enhanced surface acidity because of: (i) the formation of new Bronsted acid sites; and (ii) the strengthening of some acid sites of the parent zeolite. This is ascribed to the proton attack of the zeolite surface by the chemisorbed H+F- ion pair. Under these preparation conditions, the zeolite structure is fully preserved. Such enhanced acid properties are traduced by a significant increase in the yield of methyl tert-butyl ether (gas phase reaction).