A model describing the relationship between the orthopositronium 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 orthopositronium. The predictions of the present model are also confronted against an already known experimental result. (C) 2004 American Institute of Physics.