The lifetime Of ortho-positronium in solid naphthalene shortens with the rise of pressure as a result of diminishing the free spaces in which it is trapped. At the pressure approximate to100 MPa the size of these spaces is too small to accommodate the Ps atom (no energy level in the well), and the intensity of ortho-Ps component. initially weakly dependent on pressure, abruptly drops to zero. The narrow range in which ortho-Ps disappears indicates uniformity of Ps-accommodating voids. The nonmonotonous dependence of free positron lifetime is discussed in the framework of the blob model. (C) 2004 Elsevier B.V. All rights reserved.