The use of a large probe molecule like di-tert-butyl-pyridine (DTBPy) is showed to be adequate to characterize the acid shes located at the external surface of zeolites. Infrared spectroscopy has been revealed as a good technique to monitor the adsorption of the base by following the N-H+ stretching band at 3370 cm(-1) of the protonated amine. In this way, zeolites with 10 and 12 member ring (MR) pores with uni-, bi-, and tridirectional system of channels have been studied. On some samples, the relative amount of external acid sites has been changed by modifying the crystal size or by selective Al leaching of the surface. The external surface acidity measured by this technique correlates well with the cracking activity for large reactant molecules that cannot penetrate inside the pores of the zeolites.