Southern pine, Douglas-fir, and aspen wood flakes were acetylated with acetic anhydride vapor and compared with flakes acetylated with liquid acetic anhydride diluted with xylene. The rate of acetylation was much lower for the vapor than for the liquid phase reaction. Acetylation weight percent gains above 20 were achieved by both methods. Flakeboards made from both types of flakes absorbed much less water, both in water soaking tests and when subjected to humid air, and swelled at a lower rate and to a lower extent than did control boards. At low weight gains of vapor phase acetylation, the rate and extent of swelling were higher than those found for the controls.