Coke formation over USY and steamed USY zeolites was investigated by in-situ FTIR spectroscopy, i.e., an IR cell which simultaneously serves as a reactor using n-hexane cracking as a test reaction and which is attached to a gas chromatograph. The rate of coke formation and catalyst de-activation increased with increasing the concentration of acid sites. The de-activation during n-hexane cracking can be divided into two periods. During the first stage of coke formation, the activity decreased linearly with coke formation. At this stage, coke was predominantly deposited on the Bronsted acid site inside the zeolite structure. In the second stage, which met with a higher coke content, the activity was much less affected by further coke formation. At this stage, coke grew mainly outside the zeolite pores.