Chain interpenetration in polymer glasses was investigated by a new strategy combining preparation of an isotopically enriched polymer blend with analysis by ¹H dipolar filtered solid-state NMR under fast magic angle spinning (MAS). The fast MAS at 25 kHz induces crucial changes in spin dynamics and selectively isolates the proton signal of hydrogenous PS chains in which protons are sufficiently close to the ²H nuclei of deuterated PS chains.  Chain interpenetration and the intimacy of segment mixing in isotopically enriched blends were qualitatively characterized.