This work deals with the synthesis of high-impact polystyrene (HIPS) using a novel cyclic multifunctional initiator the diethyl ketone triperoxide (DEKTP), analyzing its effect on the prepolymerization step and its influence over the HIPS final properties. DEKTP showed a low selectivity toward grafting reactions at any of the concentrations of initiator and temperatures evaluated, and when it was used under a temperature ramp (120-130 degrees C, heating rate = 2 degrees C/min), the synthesis of HIPS was possible with an adequate development of morphology and impact strength (i.e., 67 J/m) depending on DEKTP concentration. (C) 2009 Wiley Periodicals, Inc, J Appl Polym Sci 114: 3198-3210, 2009