The effect of reprocessing two mixtures: recycled high impact polystyrene (HIPS) with polybutadiene (PB); and HIPS with styrene-ethylene-butylene-styrene (SEBS) have been studied in this article. To simulate recycled HIPS, we reprocessed virgin HIPS through five cycles. The virgin HIPS, the recycled HIPS, and the mixtures have been mechanically and rheologically characterized after the various cycles of reprocessing to evaluate their corresponding properties and correlate them with the number of cycles undergone. To widen our injection simulation analysis by computer (CAE: Computer Aided Engineering) of these new materials with additives, it was necessary to determine the viscosity using a mathematical model, in this case the Cross-WLF, to determine the relevant parameters. Our results show that tensile strength increases, while the lengthening and viscosity decrease, as the number of reprocessing cycles increases. If we mix PB or SEBS with degraded HIPS, the mechanical properties of the virgin material are recovered and the rheological behavior maintains the viscosity values for the mixture with PB but these diminish notably for the mixture with SEBS. The results on mechanical properties show that the mixture with SEBS behaves better than the mixture with PB as the number of cycles increases. The viscosity of the mixture with SEBS diminishes more than the viscosity of the mixture with PB as the number of reprocessing cycles increases. (C) 2010 Wiley Periodicals, Inc. J Appl Polym Sci 120: 2400-2410, 2011