Proton NMR imaging techniques were used to characterize a silicone rubber, intended mainly for biomedical applications, modified by an "in situ" polymerization. The study was aimed at the investigation of the effect of the polymerization on the silicone properties and of the dispersion of the guest polymer inside the host matrix, Before the onset of the reaction, the silicone, swollen either in water or in toluene, was characterized. Any source of inohomogeneities such as air bubbles or defects was localized and measured, and the geometry was recognized. By comparing the signal intensity from images collected before, during and after the polymerization reaction as well as the T-1 and T-2 relaxation time values, the guest polymer was found almost homogeneously dispersed in the host matrix, without preferential locations and/or agglomerates. The silicone network did not appear significantly altered by the presence of the guest polymer. The occurrence of the "in situ" polymerization reaction was proved by infrared spectroscopy.