PVC/PS blends are obtained through a reactive extrusion-polymerization method by the absorption of a solution of styrene monomer, initiator, and a crosslinking agent in commercial suspension-type porous polyvinyl chloride (PVC) particles, forming a dry-blend with a relatively high monomer content. These PVC/styrene dry-blends are reactively polymerized in a twin-screw extruder in the melt state. They do not contain monomer residues as detected by GC. The transparency, fracture surface morphology, thermal stability, rheology and static and dynamic mechanical properdes of these blends are compared to physical PVC/PS blends at similar compositions. Owing to the high polymerization temperature (180degreesC). short PS chains are formed in the reactive extrusion process. These short chains are dispersed both as a separate phase of similar to2 mum particles (recognized by SEM) and also as molecularly dispersed chains enhancing plasticization and compatibilization. The molecularly dispersed short PS chains tend to plasticize the PVC phase, reducing its melt viscosity and glass transition temperature. The content of the short PS chains forming the dispersed separate PS particles is too low for DMTA to detect a separate T-g. Thus, reactively ex-truded PVC/PS blends exhibit single T-g transitions at lower temperatures compared with the neat PVC. Migration of the PVC's low-molecular-weight additives (lubricants and thermal stabilizer) to the PS phase is observed in the physical PVC/PS blends, causing antiplasticization. of the PS phase. This results in both reduction of the T-g and an increase in the thermal stability of the PS phase in the physical PVC/PS blends. Comparing TGA thermograms of reactively extruded and physical PVC/PS indicates that the PS formed in the extruder is different from the commercial PS. This can stem from various chemical reactions that can take place in the studied reactive polymerization process. (C) 2004 Societv of Plastics Engineers.