PURPOSE: To extend the lifetime of an absorbent in a recycle stream treatment vessel and prevent the lowering of efficiency of a free radical initiator during polymerization of monovinyl aromatic compounds by removing high boiling point volatile compounds using heat exchangers, devolatilizers and filter beds.

CONSTITUTION: A monovinyl aromatic monomer stream is placed in a reacter vessel system under the polymerization condition and a partial purified stream is formed by separating at least one portion of volatile compounds from the solids and the non-volatile liquids in said stream and at the same time, a purified recycle condensate is formed by condensing at least one portion of said volatile compounds in a condenser and subsequently removing undesired compounds and recycled into the reacter vessel system. On the other hand, a purified stream is formed by further devolatilizing said partial purified stream under a higher vacuum than the previous devolatilization and an additional portion of volatile compounds is removed from the process and the purified polymer stream is flowed into a finishing system.