High-pressure extractions and delignification of red spruce have been carried out using acetic acid-water, acetic acid-carbon dioxide, and acetic acid-water-carbon dioxide mixtures. Effect of solvent composition (from 0 to 100% acetic acid), pressure (from 1.4 to 25 MPa), temperature (from 120 to 180 degrees C), and extraction time (from 0.5 to 3 h) has been studied. Extent of delignification has been determined by chemical (acid insoluble fraction), spectroscopic (FTIR), and microscopic (SEM) evaluations. With acetic acid-water binary mixtures containing 73 mol % (90 vol %) acetic acid, high delignification levels (in the range from 70 to 95%) were achieved with relatively high yields (60 to 50%, respectively). The physical properties of the pulp obtained with this mixture were comparable to conventional-pulps. Breaking length and burst index were around 7.8 km and 4.3 kPa m(2) g(-1), respectively. In extractions with acetic acid-carbon dioxide and acetic acid-water-carbon dioxide mixtures, extent of dissolution and delignification levels were, however, lower.