In the present invention, a process is provided for producing high impact polystyrene of improved heat resistance. According to the invention, after phase inversion of the rubber-polystyrene system, a slurry of PPE in styrene is added in situ to the process. The PPE slurry in styrene contains greater than 15 wt. % of PPE and is introduced to the process at point past rubber phase inversion, preferably at point where total polymer solids is greater than 40 wt. % of the total weight of the polymerization mixture. Preferably, four continuous stirred tank reactors in series are used in the process and preferably the slurry of PPE in styrene monomer is introduced to the third CSTR.