Extraction characteristics are shown for trioctylphosphine oxide (TOPO) dissolved in alkane for recovery of acetic acid from dilute water solution and hardwood-derived hemicellulose extracts. The recovery of acetic acid with TOPO is significantly influenced by the pH in the aqueous phase and lightly affected by temperature. In a one-stage extraction, 76.0% of the acetic acid could be extracted below pH 3. The yield of fractional extractions increase with increasing TOPO concentration in alkane and with increasing acetic acid concentration in the aqueous phase. It was found that for dilute extractions carried out at 708C and pH 1, the solvent extraction is effective at 37% TOPO in alkane (w/w) and that little improvement is realized by further increases in TOPO concentration. Partition coefficients for green liquor and hot water extracts ranged between 2.0 and 2.5 at the tested conditions. Fermentation of hemicellulose extracts that had been treated with TOPO for removal of acetic acid was tested to determine whether TOPO processing resulted in any positive or adverse affects on the microbial activity. Fermentation of TOPO-treated green liquor hemicellulose extract with Pichia stipitis resulted in improved ethanol production relative to untreated extract. Accordingly, placement of TOPO extraction after hydrolysis and prior to fermentation is optimal for acetic acid recovery and maintenance of fermentation rates.