Wood welding is an environmentally-friendly and very quick technique to yield wood joints in just a few minutes and without using any adhesives. The only limitation of welded wood is that the joint is suitable only for interior use. Exterior use, or use in an environment with varying humidity requires water resistance of the welded joints. An investigation was performed to determine the effects of welding parameters and wood properties on water absorption in the weldline and how to reduce it through controlling the production parameters. The influences of welding pressure, welding time, and heartwood/sapwood on water absorption in the weldline of Scots pine (Pinus sylvestris) joints were investigated. Specimens composed of two pieces of heartwood or sapwood, each of dimensions 200 mm x 20 mm x 20 mm, were welded together to form specimens of dimensions 200 mm x 20 mm x 40 mm. The specimens were allowed to stand in 5-mm-deep tap water and then they were taken out of the water one at a time and scanned in 10-min intervals until the first crack appeared in the weldline. An X-ray Computerized Tomography scanner was employed to monitor water movement and density change in weldlines during water absorption-desorption. All three evaluated parameters showed significant effect on water absorption. Samples of heartwood welded by 1.3MPa welding pressure and 1.5 s welding time showed the lowest water absorption. (C) Koninklijke Brill NV, Leiden, 2011