This study was conducted to determine if thickness swelling of flakeboards, which is much greater than that of plywood or solid wood, can be improved by chemical modification of the constituent wood flakes prior to board manufacture. The chemical treatment involved acetylation of oven-dry aspen ring flakes with a 50/50 mixture of acetic anhydride and xylene. Thickness swell of treated and control samples was measured by either immersion in water for periods of up to 10 days or exposure to 90per thousand relative humidity for periods of up to 20 days. Treated samples exhibited one-sixth to one-seventh the thickness swelling of controls for the water-soak test. Similar trends were observed for humidity exposure.