A programme of experiments is used to test the validity of the energy-balance model for rubber particle cavitation(1,2). The yield behaviour of a transparent grade of rubber-toughened poly(methyl methacrylate) (RTPMMA) is investigated over a range of temperatures, in tension, compression and flexure. Differences between yield in tension and yield in compression are shown to affect flexural yield behaviour, and in particular the position of the neutral plane, which shifts towards the compression surface when the material cavitates in tension. The energy-balance model is used to calculate when cavitation is energetically possible, and the experimental observations at each temperature are compared with the predictions of the model. It is found that the release of energy is a necessary but not sufficient condition for cavitation, and that a small energy barrier at small void sizes exerts a controlling influence upon the cavitation process.