Supercritical Fluid (SCF) extraction was used to selectively remove lignin from yellow poplar wood (Liriodendron tulipifera L). The process may serve as an alternative to conventional delignification techniques practiced by the pulp and paper industry. Extraction experiments were performed by flowing supercritical ammonia-water mixtures through a fixed bed of poplar wood. Extraction of cellulose, hemicellulose, and lignin was determined as a function of time (0-3 h), solvent composition (0-20 wt% water), temperature (160-200 -degrees-C), and pressure (136-272 atm). The percentage of water in the solvent and temperature were found to be the most important variables in determining extraction efficiency. A simple model involving these variables was developed. The most effective extraction was obtained using 20 wt % water in ammonia at 272 atm and 200 -degrees-C, at these conditions about 70% of the lignin, 50% of hemicellulose, and 15% of the cellulose initially present in the wood were extracted in one hour.