The phase reaction conversion (PRC) headspace gas chromatographic (HSGC) technique was employed to develop a method for the determination of the content of carboxyl groups in wood fibers. Treatment of the wood fibers using hydrochloric acid was applied to convert carboxyl groups to carboxyl acids. Bicarbonate solution was then used to react with carboxyl acids on the treated fibers in the headspace of a testing vial to form carbon dioxide that was analyzed by a thermal conductivity detector using gas chromatography. The conversion reaction was conducted at 60 degreesC for about 10 min to achieve near complete conversion. The contribution to the GC detector signal from carbon dioxide formed by residual hydrochloric acid on wood fibers can be accounted for by the known experimental parameters. The effect of carbon dioxide in the headspace of the testing vial air was calibrated or can be eliminated by purging the testing vial using nitrogen before conducting experiment. The measured contents of carboxyl groups in eight wood fiber samples were in good agreement with those measured by a titration method. The present method is accurate, rapid, and automated.