As a result of climate change, the species structure of forest stands is gradually changing when the growing share of core species of deciduous trees at the expense of conifers places demands on its future efficient processing into final products. This study evaluates the influence of the surface modification of the heartwood of oak (Quercus robur) with five selected chemical modifiers on the strength of the glued joint with the used one-component PUR adhesive for interior and exterior structural applications. Samples of test bodies produced according to the standards were exposed to the given environment and subsequently subjected to a test using the method of determining the longitudinal shear strength under tensile stress. By applying a chemical modification to the surface layers of the glued surfaces of oak wood, the tannins in the surface layers were neutralized, increasing the adhesion of the glued surface. The resulting values of the strength of the glued joints with the applied chemical modification of the heartwood surface also confirmed this.