Southern pine and aspen wood flakes were chemically modified by reaction at ca. 55°C with ketene in the absence of solvent. Reactions were relatively slow, with weight gains of up to 17per thousand and 20per thousand, respectively, obtained. Acetyl content correlated with weight gain only up to the 12per thousand level. Water and solvent extraction of ketene-modified southern pine and aspen flakes showed very little loss in acetyl. Flakeboards made from southern pine and aspen flakes treated with ketene showed a greatly reduced rate and extent of swelling resulting from liquid water sorption as compared to control boards. Similar results were obtained in swelling tests done in water vapor. Ketene modification had a much greater effect on improving dimensional stability properties of aspen flakeboards than on southern pine flakeboards.