Poly(methylphenylsiloxane)-poly(methyl methacrylate) interpenetrating polymer networks (PMPS-PMMA IPNs) were prepared by in situ sequential condensation of poly(methylphenylsiloxane) with tetramethyl orthosilicate and polymerization of methyl methacrylate. PMPS-PMMA IPNs were characterized by infrared (IR), differential scanning calorimetry (DSC), and Si-29 and C-13 nuclear magnetic resonance (NMR). The mobility of PMPS segments in IPNs, investigated by proton spin-spin relaxation T-2 measurements, is seriously restricted. The PMPS networks have influence on the average activation energy E-a,(av) of MMA segments in thermal degradation at initial conversion.