Positronium self-trapping in liquids is frequently described through the bubble model in order to account for the observed decay characteristics, namely, the enhanced pick-off lifetime and the details of the angular correlation of the decay gammas. This model is improved upon in the present paper by modifying the idealised picture of a sharp bubble radius and thereby a formalism is developed which is not only more realistic but also provides elegant analytic solutions. (C) 1997 American Institute of Physics.