<p>Wood absorbs carbon dioxide from the atmosphere, and by being converted into wood-based materials, stores carbon in the long term, thereby contributing to the mitigation of global warming. Wood production relies on adhesives, making a detailed understanding of the adhesive interface essential to ensure performance. In this study, the confocal laser scanning microscope used was capable of simultaneously detecting three distinct fluorescence wavelengths. This setup allows clear visualization of the distributions of melamine–urea–formaldehyde resin (MUF) and polymeric methylene diphenyl diisocyanate (pMDI) within plywood, enabling precise differentiation of each component and providing detailed insights into their spatial distributions. Microscopic analyses revealed that MUF infiltrates intercellular spaces and ray tracheids, extending outward from the adhesive interface. In contrast, pMDI is observed not only near the adhesive interface, but also in the cell lumens located farther from the interface. The authors propose that the penetration of water-based, low-viscosity adhesives, such as MUF, is facilitated by pathways that originally served as water conduits in living wood, enabling efficient transport through wood structures. Conversely, the penetration of hydrophobic, viscous adhesives, such as pMDI, into such narrow spaces is limited. However, pMDI can penetrate through openings in areas where cell lumens are exposed on the cut surface of the wood and spread into the cell lumens. The fundamentally distinct penetration mechanisms of MUF and pMDI allow them to function without interfering with each other. This complementary behavior suggests that a hybrid adhesive system comprising both materials could exploit their respective strengths and enhance bonding performance while reducing the limitations of each adhesive individually.</p>