The positron lifetime spectra have been measured as a function of exposed air pressure for synthetic zeolite 13X and 3A
and as a function of calcination temperature for zeolite 10X.It is found that the adsorbed gases have a strong effect on positron annihilation.Moreover
the positron annihilation parameters varysignificantly with the amount of water existing in the zeolite cages. With the dehydration the free volume inside cages bacame larger