Three interpenetrating networks based on polymethylphenylsiloxane (PMPS) and polymethylmethacrylate (PMMA) with different compositions were prepared. They were investigated using differential scanning calorimetry (DSC) and high-resolution solid-state C-13 NMR techniques, in order to study the heterogeneous character of the domains involved in these materials at the nanoscopic scale. DSC observed broad glass transitions of the PMPS chains. C-13 cross-polarization (CP) experiments clearly showed that the aromatic PMPS carbons have a rate of CP which is composition dependent, indicating that PMPS and PMMA chains are interpenetrating at a very low spatial scale. These results were supported by T-1p(H-1) and C-13 line width measurements as a function of temperature. Moreover, T-1p(H-1) measurements indicated that PMMA chains are rather homogeneously distributed whereas PMPS chains belong either to interpenetrating domains or to PMPS-rich domains.