Thermal modification of wood results in improved dimensional stability and increases the end-use possibilities of wood. Modification under saturated steam is reported to result in higher performance when compared to more traditional thermal modification methods.This study analyses the chemical and ultrastructural changes, as well as water vapour sorption properties of Scots pine modified thermally in a high-pressure reactor under saturated steam. The aim is to reveal important chemical and sorption-related changes in wood modified under saturated steam. Chemical composition, water vapour sorption properties, accessibility and concentration of cellulosic hydroxyl groups, as well as evolution of cell wall are discussed. At a temperature of 180 °C, clear cell wall delamination and distortion were observed. In nanoscale, the results indicated opening of microfibril bundles, which leads to higher surface area and theoretically, a higher accessibility. However, a decrease in the equilibrium moisture content and accessibility of both extracted and unextracted samples were observed, but the decrease was less obvious in extracted samples.Hence, it was concluded that extractives and degradation products play an important role during thermal modification by blocking porosity and therefore decreasing accessibility and reducing sorption of thermally modified samples. The changes in hysteresis behaviour after extraction also support this outcome.