Solid electrolyte interphase (SEI) plays an important role in determining electrochemical performances of Li-ion batteries. The ideal SEI layer protects the electrolyte from being further reduced on the electrode surface and allows Li-ion diffusion in and out of electrodes without any consumption. However, degradation of the SEI layer over time, which contributes to the thickening of the SEI layer, is a leading pathway for gradual capacity fade. In this study, a hyperpolarized (HP) Xe-129 nuclear magnetic resonance (NMR) technique was applied for the first time to probe changes in porosity and connectivity in Si nanoparticle composite electrodes as a result of the SEI formation. Nanopores are present in nanocomposite electrodes as a result of aggregation of the constituting nanoparticles. The connectivity among nanopores greatly affects the ion transport property of the electrode materials, which has a substantial influence on the overall energy output of Li-ion batteries. In this work, information on thickness, uniformity of the SEI layer, and connectivity of the pores in the composite electrodes upon growing SEI was obtained from the analysis of temperature-dependent HP Xe-129 NMR spectra. Such information is useful for gaining a better understanding of the degradation mechanism of SEI. This study demonstrates that HP Xe-129 NMR is a potentially unique tool in probing the porosity and connectivity changes in porous practical electrodes during electrochemical cycling.