A process for producing high impact polystyrene comprising introducing to a reactor a partially-polymerized mixture comprising at least one vinyl aromatic monomer, an elastomer and reacted vinyl aromatic monomer wherein the partially-polymerized mixture has not undergone phase inversion; polymerizing the partially-polymerized mixture in the reactor to the phase inversion point to form a phase-inverted mixture; recovering a portion of the phase-inverted mixture from the reactor wherein the phase-inverted mixture comprises high impact polystyrene; and introducing another portion of the phase-inverted mixture to another reactor.