The work described in this paper demonstrated that acetylation of pine wood up to 16.4 weight percent gain has a positive effect on the compression strength. Higher modification levels resulted in a reduction of the compression strength of wood, compared to the untreated wood. A comprehensive investigation of the effect of molecular size of the substituent group of softwood modified with linear chain carboxylic acid anhydrides, namely acetic, propionic, butyric, valeric, hexanoic, upon the compression strength has also been performed. It was found that the improvement of compression strength imparted by chemical modification is independent of the degree of bulking of the cell wall, but correlates well with the degree of substitution of the cell wall hydroxyl groups. (C) 2010 Elsevier Ltd. All rights reserved.