Dynamic mechanical measurements were carried out on rubber-toughened poly(methyl methacrylate) (RT-PMMA) under superimposed static tensile and compressive stresses, in the temperature range -40 to 10 degrees C, which includes the glass transition of the rubber phase. Under compression, where the dynamic loss curves were very reproducible, the tan delta peak due: to the rubber particles shifted to higher temperatures with increasing superimposed stress. Under tension, by contrast, the loss curves were much more variable, in some cases splitting into two peaks, and the peak temperature was no longer a function of the superimposed stress. This behaviour is consistent with incipient cavitation in a proportion of rubber particles under tensile stress. It is concluded that the transition from reproducible to variable dynamic mechanical behaviour marks the onset of cavitation in the rubber phase. The specimen-to-specimen variations observed under superimposed tension appear to be due to differences in strain distributions within and between rubber particles. The procedure provides a method for measuring the resistance of rubber particles to cavitation in response to mechanically and thermally generated stresses, and for distinguishing the cavitation event from subsequent dilatational yielding. (C) 2000 Elsevier Science Ltd. All rights reserved.