Pore size of several porous materials such as zeolites (MCM-41 and MCM-48) and highly fined grain aggregates of zirconia was estimated by means of positron annihilation lifetime measurement. Observed lifetimes of ortho-positronium (over -20ns, τ4 and τ5) correspond to the meso-pore sizes assessed by conventional porosimetries due to a nitrogen physisorption and a mercury porosimetry. The long-lived lifetime components included an unassigned lifetime component (τ5), which was not detected by the other porosimetry. The observed ortho-positronium lifetimes are correlated to the mean size of pores. A relation between relative intensities of the long-lived ortho-positronium components (I4 and I5) and specific surface areas is discussed and a variation of a numerical concentration of pores is correlated to the change of the particle sizes of the zirconia aggregates.
