The lifetime of para-positronium in raw materials of MCM-41 class was measured as a function of temperature. It was found that the lifetime shortens when the template filling is removed and empty pores are created. Rough data are given also for phase transition in n-nonadecane. The electron contact density eta on the positron bound into positronium atom in the condensed media, determined in experiments, seems to be related more to the size of positronium trap than to the electronic properties of the medium. The role of contact density eta in the estimation of ortho-positronium lifetime in the bulk is discussed. (C) 2003 Elsevier Science B.V. All rights reserved.