It has long been known that the mechanical energy of plastic deformation tranforms partly into heat (ratio βint) which can cause noticeable temperature rise under adiabatic conditions. Less is known about the rate of conversion of these quantities (ratio βdiff). High strain rate deformation of metals was recently investigated by Mason et al. (1994) and Kapoor and Nemat-Nasser (1998). The former investigated the rate ratio βdiff and observed a strain and strain rate dependence. The latter investigated βint and concluded it is constant and equal to 1. Temperature measurement was made using infrared techniques. We investigate the thermomechanical behavior of glassy polymers (PC) deformed at strain rates ranging from 5000 to 8000 s−1. The temperature is assessed using small embedded thermocouples whose applicability to transient measurements has been recently revisited (Rittel, 1998a). Our results show a definite dependence of both β factors on the strain and strain rate. We also observe that whereas the overall ratio (βint) of the converted energy remains inferior to 1 as expected, the rate ratio (βdiff) may reach values superior to 1 at the higher strain rates. This original observation is rationalized in terms of a decrease of the stored energy of cold work which corresponds to the softening regime of the stress strain curve. This additional energy transforms into heat thus causing the observed values of βdiff. While the present observations apply to a glassy polymer, it can reasonably be assumed that they apply to the broader context of strain softening.