The present work aimed to explore a new approach for more advanced in situ compatibilization through studying the effect of supercritical carbon dioxide (scCO(2)) as a green processing medium on in situ compatibilization of polypropylene/polystyrene (PP/PS) blends via reactive extrusion. The effectiveness of the in situ compatibilization with scCO(2) on the phase dispersion and mechanical properties was studied via comparison with that of the other approaches of thermal blending without scCO(2), thermal blending with scCO(2) and in situ compatibilization without scCO(2). In the in situ compatibilization with scCO(2), trimethylolpropane triacrylate (TMPTA) and dibenzoyl peroxide (BPO) were applied as polyfunctional monomer and radical initiator, respectively. Fourier transform infrared spectroscopy (FTIR) was used to confirm the formation of the block and/or graft copolymers (PP-b-PS and/or PP-g-PS) during reactive extrusion and further to study the effect of scCO(2) on the yield of the interfacial modifiers. Scanning electron microscopy (SEM) was applied to observe the phase dispersion of the PP/PS blends prepared via different approaches and thus to analyze the functions of scCO(2) in improving the phase dispersion in the in situ compatibilization. Impact tests were carried out with samples prepared via different approaches without and with thermal annealing to confirm the effectiveness of in situ compatibilization with scCO(2). The results showed that the interfacial modifiers were in situ formed during reactive extrusion, and that the yield was increased with scCO(2). It was further found that scCO(2) could improve the phase dispersion and thus the mechanical properties of the PP/PS blends prepared via in situ compatibilization through two functions: scCO(2)-assisted phase dispersion and scCO(2)-promoted in situ compatibilization. Therefore, the better phase dispersion of the PP/PS blends prepared via in situ compatibilization with scCO(2) could be attributed to the two functions of scCO(2) as well as a basic thermal blending and a primary in situ cornpatibilization. Thus, compared to the other approaches, in situ compatibilization with scCO(2) has proved to be a more effective approach in improving phase dispersion and enhancing mechanical properties. (C) 2016 Elsevier B.V. All rights reserved.