The hysteretic thermal response of two polymers, polymethyl-methacrylate (PMMA_REF) and a tougher modified PMMA + N-methyl glutarimide (PMMA-MOD) has been investigated under compressive cyclic loading at high stress levels. The modification increases the ability of PMMA_REF to undergo plastic deformation. This work characterizes the thermomechanical. response of both polymers at high cyclic stress levels (of the order of sigma(y)), that were hot previously investigated. The comparison clarifies the contribution of increased chain mobility on the hysteretic heating phenomenon, with emphasis on the nature of a significant exothermal peak that was previously observed in commercial polycarbonate (PC). In this work, both PMMA_REF and PMMA-MOD were subjected to compressive cyclic loading, and the temperature was continuously monitored until specimen failure. The experimental results show that, despite the higher toughness PMMA-MOD, the two polymers fail in a very similar fashion, both in terms of temperature rise and bulging failure mode, without the initial thermal peak that was observed in commercial PC. It thus seems that the unique thermomechanical response of PC is neither related to its plastic flow properties, nor to its ability to undergo high cyclic stresses. It is suggested that a cyclic stress induced/related exothermal phase transition might be responsible for the thermal peak in this material. (C) 2003 Elsevier Science Ltd. All rights reserved.