Results of the dynamic mechanical behavior of atactic polystyrene (PS) and high-impact polystyrene (HIPS) for temperatures between 300 and 425 K at a frequency of the order of 50 kHz are presented. The storage Young＇s modulus, (E＇), of the HIPS is lower than the PS value, being the relationship between them a function of the rubber phase volume fraction, independent of the measurement frequency. The glass transition temperature (T-g) of HIPS is shifted to lower temperature in respect to the PS. The gamma relaxation appears at 308 Kin PS at 50 kHz, while it seems to move toward lower temperatures in the HIPS. Both shifts are attributed to the presence of mineral oils in the HIPS. The values of E＇, T-g, and the temperature of the gamma relaxation at 50 kHz are discussed within the scope of the theory of viscoelasticity. Finally, the effect of thermal treatments, using different annealing times, on the behavior of both materials is shown. (C) 2000 John Wiley & Sons, Inc.