Cyclic N-methylol compounds such as 1,3-dimethylol-4,5-dihydroxyethyleneurea (DMDHEU) have been used to modify wood and prevent negative effects related to the uptake of moisture. However, the changes in the sorption behavior of wood by treatments with DMDHEU and its derivatives are not fully understood. In the present study, wood blocks were treated with DMDHEU, ether-modified DMDHEU and diethyleneglycolated DMDHEU in order to study the factors that control the changes in moisture uptake in the hygroscopic range (0–95% RH). Dimensional changes of wood blocks during water soaking cycles suggested that the treatments caused a permanent cell wall bulking, whereas the swelling restraint by cross-linking of adjacent cell wall polymers was not permanent. However, the changes in water vapor sorption were not only a result of the cell wall bulking effect that reduced the space in the cell wall to accommodate water. The N-methylol compounds within the wood also provided additional sorption sites, but there was no correlation between absorbed water and accessible OH groups. It was speculated that the co-condensation of the N-methylol compounds with wood polymers had a significant effect on the sorption of the treated wood. At elevated RH, pure resins that were formed by self-condensation took up large quantities of moisture. However, when the N-methylol compounds were heat-cured within the hierarchical structure of wood, the moisture uptake of the treated wood at elevated RH was even lower compared to unmodified wood. Furthermore, the covalent bond formation between wood and resin prolonged the attainment of an equilibrium moisture content.