Abstract: Positron annihilation measurements have been carried ont for Y zeolites with different types of cations, faujasites with different Si/Al ratios and zeolites with different structures. The o-Ps annihilation lifetimes are closely correlated with the variations of the electrostatic field in the zeolites. The o-Ps lifetimes are shortened when the electrostatic field in the zeolites is increased. Positron annihilation data show that the electrostatic field of cation-exchanged Y zeolites varies in the order of KY<NaY<BaY<CaY<CeY. As the Si/Al ratio of the faujasites increases the annihilation lifetimes are decreased, because the electrostatic field in the zeolite is increased owing to the diminished shielding from the neighboring cations. For zeolites with different structures and pore sizes the o-Ps annihilation lifetime on the surface is affected mainly by the variation of the electrostatic field in the zeolites, while the o-Ps annihilation lifetime in the cages depends distinctly on the zeolite pore size. The o-Ps annihilation lifetime in the cages is directly proportional to the average pore size of the zeolites. This linear relationship between the o-P annihilation lifetime and the average pore size may be utilized to estimate quickly the pore size of zeolites with unknown structure.