Heat-treated and matched boards of Scots pine (Pious sylvestris), Norway spruce (Picea abies) and Birch (Betula pubescens) were dried at 60 C and submerged into water, allowing for water absorption in the longitudinal direction during two weeks in an indoor climate. Heat treatment was performed according to the Thermo-Wood process at 170 degrees C and 200 degrees C. Boards from pine and spruce contained both sapwood and heartwood. During water absorption, computer tomography scanning (CT scanning) was performed intermittently in order to measure the ascent of capillary water. Longitudinal water absorption in heat-treated pine sapwood was substantially lamer than in untreated sapwood. In pine heartwood, the ascent of water was low in heat-treated as well as in untreated boards. Spruce showed low water absorption in sapwood and heartwood in heat-treated as well as in untreated boards. In birch, water absorption was lower in heat-treated wood than in wood dried at 60 degrees. SEM studies of the anatomical microstructure, the pits and pit membranes, were performed on heat-treated as well as on untreated material. SEM studies revealed damage in heat-treated and dried pine sapwood mainly in pit membranes in the fenestriform crossfield pits connecting longitudinal tracheids with radial ray parenchyma cells. This damage is believed to play an important role in explaining the differences in water absorption between pine and spruce, since the piceoid crossfield pits in spruce seemed to be unaffected by heat treatment. In comparing the three different treatment temperatures in birch, no striking, visible differences were found that could shed light on the observed large differences in capillary water absorption