<p>Understanding the molecular origin of stress softening in silica-reinforced polydimethylsiloxane remains a challenge. Here, we apply in situ elongational nuclear magnetic resonance (NMR) spectroscopy to selectively probe the polymer matrix under deformation. This allows us to address two key questions: how silica nanoparticles influence microscopic polymer deformation and the critical contribution of the interfacial layer to stress softening. By exploiting differences in chain dynamics, we selectively detect protons in the polymer network strands between topological constraints and the interfacial layer. Combining in situ NMR with numerical modeling based on the nonaffine tube model helps us decouple contributions of network strand deformation and orientation to the magnetic resonance signal. We show that nanoparticles significantly influence network deformation via strand reorientation at small strains. However, the strain-independent interfacial fraction indicates a minor role for the interfacial layer in stress softening during quasistatic loading-reloading tests.</p>