Confocal microscopy has been used to investigate the behavior of urea-melamine-formaldehyde (UMF) resin after being sprayed onto wood fibers. Samples of wood fibers were dyed with Toluidine Blue O (TBO) to create a contrast between the resin and the fiber when studied under a confocal laser scanning microscope (CLSM). The resin droplet penetration into the fiber cell wall was evaluated from the three-dimensional images generated by the CLSM. Experimental conditions were varied to determine their influence on the penetration of the resin into the cell wall. The Washburn equation was used to model the penetration process. The model takes into account variations of the resin viscosity with temperature and time. The penetration rate was found to be related to resin viscosity. The penetration depth can be as low as 1 mu m if the resin is applied on a dry spruce fiber in a drum mixer. The maximum penetration depth corresponds to the thickness of the cell wall, which can vary from 10 to 20 mu m for spruce. Further work should lead to the development of a more effective resin that could be used to make boards with the same performance but less resin.