Changes in the acidic properties and catalytic activity of MFI-type zeolite catalysts caused by dealumination in a steam atmosphere were examined. The reaction rate constant of the cracking of n-hexane was used as an index of the catalytic activity. The zeolite catalysts were exposed to a steam atmosphere for 3-20 h under a steam partial pressure of 1.0-101 kPa, and a temperature of 773 K. The amounts of acid sites induced by each kind of aluminum atoms inside and on the outer surface of the steamed zeolite crystals were measured separately by TPD methods with ammonia and 2,6-diethylaniline. Dealumination in a steam atmosphere was found to proceed as follows : first, tetrahedral aluminum atoms are extracted out of the framework and transformed into partially distorted octahedral aluminum atoms, which also induce acid sites. These atoms move to the outer surface of the zeolite crystals. During this movement, a part of them is pushed out of the crystals and turn into perfectly octahedral ones, which never induce acid sites. The remaining partially distorted octahedral aluminum atoms arrive at the outer surface of the crystals, and then they are pushed out of the crystals and turn into perfectly octahedrally coordinated aluminum atoms. With the progress of the dealumination, the catalytic activity increased at the beginning of the process, reached a maximum value, and then decreased. This phenomenon was considered to be dependent on the change in the amount of partially distorted octahedral aluminum atoms on the outer surface of the zeolite crystals.