  A model describing the relationship between the ortho--positronium lifetime
and the volume of a void, located in a synthetic zeolite, is analyzed. Our
idea, which allows us to take into account the effects of temperature,
comprises the introduction of a non--hermitian term in the Hamiltonian, which
accounts for the annihilation of the ortho--positronium. The predictions of the
present model are also confronted against an already known experimental result.
