Enhanced Bronsted acidity is experimentally modeled by a low-temperature adsorption of a Lewis acid (BF3) on the hydrogen form of EMT zeolite. The increase in acid strength is detected in FTIR studies of a low-temperature adsorption of carbon monoxide on the samples pretreated with BF3. The proton affinity of Bronsted hydroxyls decreases linearly with increase in the surface coverage with BF3. A structure for a surface complex, zeolite-Lewis acid, is proposed, and local and cooperative views on the mechanism of enhancing the acid strength of Bronsted hydroxyls are discussed.