In this paper the lifetime of o-Ps is discussed. It is considered that the lifetime of o-Ps in condensed molecular substances depends primarily on the volume-factors and dielectric properties of materials, but the chemical and paramagnetic quenching of o-Ps by molecules and active radicals created in the spur can not be neglected, and sometimes they affect the lifetime of o-Ps considerably. A semi-empiricial formula for estimation of lifetime of o-Ps in pick-off annihilation is proposed, it is expressed as: \I\3 = K(M\I\/\T\-\S\Voi)/e\S\Zi where K=3.08(ns/cm3) is an empirical constant; s e is the dielectric constant of material; Mr and \T\ are the molecular weight and density of material respectively; Zi and Voi are the number of valence electrons and the mole excluded volume, respectively, of atom i by which the molecules of material are constituted. An empirical formula for Voi is also proposed. By comparison between calculated and experimental \I\3 values, a discussion about the influence of the free radicals in the spur and molecules of material on o-Ps lifetime is made.

Key words: POSITRONS, ANNIHILATIONS, QUENCHING EFFECTS, DIELECTRIC PROPERTY, POSITRONIUM