The simple-to-process low-cost Friedel–Crafts alkylation reaction was used for in situ compatibilization of PP/PS blends. This reaction forms a graft copolymer at the interphase, which behaves as a compatibilizer. It was found that there is a notorious emulsifying effect due to this interphase modifier. A net particle-size decrease along with catalyst content was obtained. Great improvements in both elongation at break and toughness were achieved. This result is in agreement with the interfacial adhesion observed in the morphological analysis. These properties increase with catalyst content up to maximum values, which are reached at 0.7%.