Wood-based products can be a sustainable and more environmentally friendly alternative to traditional construction materials because of their reduced contribution to air and water pollution. An integral component of these products is often an adhesive. Because wood is hygroscopic, moisture-induced swelling in the cell walls near the wood–adhesive bond lines can lead to durability and performance issues. Unfortunately, researchers working toward improving the moisture durability of forest products are hindered by an incomplete understanding of the nanoscale mechanisms that contribute to moisture-induced swelling in wood and how chemical modifications affect the swelling. Therefore, we developed small-angle neutron scattering (SANS) into a tool that can study the 1- to 100-nm structure of unmodified and chemically modified wood cell walls and can measure the effects of moisture in this structure. In this study, SANS was used to reveal the nanostructure of a deuterium-labeled phenol-formaldehyde (dPF) adhesive infiltrated into wood cell walls. The results revealed that the dPF infiltrated the water-accessible regions between the elementary fibrils inside the wood cell walls. These results provide the new insight that adhesive infiltration into the cellulose microfibril (a bundle of elementary fibrils) may be a key to designing moisture-durable wood adhesives.