The structural change of the Y type zeolite was studied before and after dealumination by the positron annihilation technique. Four lifetime components were obtained in the NaY zeolite

while five lifetime components appeared in the USY zeoIite after dealumination. The positron lifetimes of the USY zeolite under different air pressures were measured and it was found that the longest-lived component T5 and its intensity I5 decreased strongly with the increase of air pressure. The result showed that secondary pores were formed in the USY zeolite after ion exchange and high- temperature hydrothermal treatment.