In order to determine the influence that hydroxyl group content has upon the sorption isotherm, acacia (Acacia mangium) and sesendok (Endospermum malaccense) were thermally modified for different times and at different temperatures in order to achieve differing levels of thermal modification. In a separate experiment, birch samples were acetylated to different levels of weight percentage gain. The water vapour sorption isotherms of the thermally and chemically modified wood samples were determined using a dynamic vapour sorption (DVS) apparatus. Increasing temperature and increasing time of thermal modification, or increasing levels of acetylation both resulted in a reduction in the hygroscopicity of the samples. The hydroxyl group content was determined by using the deuterium exchange technique, also using a dynamic vapour sorption apparatus. With this method, the water reservoir in the DVS is filled with deuterium oxide rather than water. Although correlation was found between the reduction in hygroscopicity and OH content for acetylated birch, no clear relationship was found for the thermally modified wood samp