The PP/PS blend is known as a typical immiscible system, andimprovement of its compatibility has been a big challenge. Theobjective of the present work is through studying the effect ofscCO2as a processing medium on the in situ compatibilization ofPP/PS blends via reactive extrusion to potentially explore a newapproach for more advanced in situ compatibilization. The PP/PSblends prepared via the approaches of thermal blending with-out scCO2, thermal blending with scCO2, in situ compatibilizationwithout scCO2were used as control samples for comparison andthus to illustrate the effectiveness of in situ compatibilization withscCO2in improving phase dispersion and mechanical properties.In the in situ compatibilization with scCO2, trimethylolpropanetriacrylate (TMPTA) and dibenzoyl peroxide (BPO) were used aspolyfunctional monomer and radical initiator, respectively. Fouriertransform infrared spectroscopy (FTIR) was used to confirm theformation of the block and/or graft copolymers during reactiveextrusion and further to study the effect of scCO2on the yieldof the in situ formed copolymers. Scanning electron microscopy(SEM) was applied to study the phase dispersion of PP/PS blendsprepared via different approaches and thus to reveal the functionsof scCO2in improving phase dispersion in the in situ compatibi-lization. Impact tests were performed with samples prepared viadifferent approaches without and with annealing to confirm theeffectiveness of in situ compatibilization with scCO2. The advan-tages of the in situ compatibilization with scCO2were illustrated