For the design of high-performing materials, the reinforcement of rubbers plays an important role. To date, carbon black and silica are most commonly used when superior reinforcement is required. On the contrary, these fillers are also known to affect the mixing and processing performance of rubber compounds negatively. Progressing from these traditional reinforcing systems, in-situ generated fillers provide a feasible way to combine high reinforcement with suitable processing conditions. Especially thermosetting resins were reported to introduce rigid particles with high reinforcing power in rubber compounds without deteriorating their processing performance. Such resins can be introduced as an interpenetrating network, where the thermoset is crosslinked during the vulcanization of the rubber. Herein, selected thermosets were evaluated regarding their curing rate and applicability for EPDM and HNBR reinforcement. Rubber/resin compounds were prepared and studied comprehensively for their curing efficiency, mechanical properties, and morphology. As a result, unique tailoring of the final properties of HNBR and EPDM-based composites was obtained by in-situ formation of reinforcing thermoset domains.