The present invention relates to a process and a device for the continuous manufacture of a polymer of an aromatic alkylene monomer e.g. styrene comprising a bulk polymerisation of the monomer and a devolatilization of the resulting polymer in at least two successive steps so as to separate the polymer from the unreacted monomer and the oligomers formed during the polymerisation. The process comprises conveying a first gas stream resulting from the first step of the devolatilization into a first zone Z1 for direct condensation and fractionation, removing a gas stream G1 comprising essentially the monomer from the top of Z1, and removing a liquid stream L1 rich in oligomers from the bottom of Z1, a portion of L1 being taken, cooled and then returned into Z1. It also comprises conveying a second gas stream resulting from the second step of the devolatilization into a second zone Z2 for direct condensation and fractionation, this zone being also fed with the other portion of L1, removing a gas stream G2 comprising the monomer from the top of Z2, and removing a liquid stream L3 comprising essentially the oligomers from the bottom of Z2. The process and the device are particularly suitable for continuously manufacturing a styrene polymer, a styrene copolymer having a high impact strength modified by grafting onto a rubber, an expandable polystyrene or an expanded polystyrene. The invention is useful for improving the separation of the unreacted monomer from the oligomers and the quality of the polymer when the unreacted monomer thus separated is recycled e.g. to the polymerisation.