Basic principles and experimental considerations of most widely used NH3-TPD in the characterization of acidic zeolites were reviewed in an attempt to shed some lights on the interpretation of experimental data. By this method up to four different acid sites can be differentiated depending on samples. It should be noted that the first peak having the lowest temperature maximum should not be considered as representing true acid sites while the other peaks may be directly related to the catalytic performances of a zeolite catalyst. Also an old but relatively unknown method for achieving better selectivity to probing true acid sites was briefly introduced.