In this paper we describe a novel NH3-TPD methodology to study the amount as well as strength of acid sites of coked catalysts. Conventional temperature programmed desorption (TPD) of ammonia cannot be applied to coked catalyst. as coke precursors removed during the temperature program falsify the ammonia signal. The effects of coke formation from different reactants, time-on-stream, and reaction temperatures on acid site deactivation were investigated. Besides acid site deactivation, characterization of coke precursors can also be revealed. The initial deactivation preferentially on strong acid sites is very fast. The concentration of free acid sites is inversely correlated well with the total concentration of coke rather than individual coke groups. Coke precursors tend to be more stable at higher reaction temperatures.