Penetration is the ability of an adhesive to move into the voids on the surface of a substrate or into the substrate itself. The cellular nature of wood can cause significant penetration of an adhesive into the substrate. The objective of this work was to evaluate the influence of the degree of condensation of urea-formaldehyde (UF) resins on the radial penetration into silver fir (Abies alba, Mill.) and hence on the distribution of resin in the wood tissue by means of microscopic determination. Three UF resins with different degrees of condensation together with extender and hardener were used as adhesive mixes and were applied onto one of the surfaces to be bonded by hot pressing parallel to the grain direction, with Safranin added as a coloring agent. Microtome slides (20 mu m thick) were cut from each joint sample, showing the bondline and the two adherends with the resin penetrated in radial direction. The depth of adhesive penetration was determined by epi-fluorescence microscopy. The results show a significant correlation between the penetration behavior and the degree of condensation (molecular size, viscosity) of the resins. The higher the degree of condensation, the lower is the penetration possibility, expressed as 'Average penetration' depth (AP). The portion of filled tracheids in the radial direction on both sides of the bondline ('Filled interphase region', FIR) however, did not depend statistically on the viscosity of the resin mix. (C) Koninklijke Brill NV, Leiden, 2010