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The real trapping potential energy U(r) of positronium in vacancy-type holes and self-sustaining bubbles in liquids is replaced by a spherical square well potential of finite energy depth U m and the scaling procedure proposed by Yu et al. is applied. Available data for the ortho-positronium lifetime in molecular crystals and in liquids are re-examined and a unique relation is found between the potential energy depth U m and the calculated radius R of the trapping site. The general form of a surface-enhanced real potential energy U(r) which is compatible with the behaviour of U m is proposed.