In this work, acrylonitrile-butadiene rubber / poly(methyl methacrylate) (XNBR/PMMA) blends were obtained through in situ polymerization of methyl methacrylate (MMA) monomer in XNBR rubber particles. Firstly, the XNBR rubber was swollen with MMA and then the polymerization was carried out in an aqueous medium, keeping the rubber particles in suspension. The results of the swelling tests indicated that mass transfer of MMA to rubber particles is more efficient in aqueous system. Fourier Transform Infrared (FTIR) analyses revealed the presence of PMMA in the XNBR phase. Thermo-gravimetrical (TG) analyses indicated that as much as 10% of PMMA was present in the blends. Scanning electron microscopy (SEM) analyses revealed that PMMA was dispersed in the XNBR phase, indicating the efficiency of the MMA polymerization inside the XNBR rubber particles.