Polychlorinated hydrocarbons are prevalent environmental contaminants whose rates of biodegradation are limited by their minimal solubilities in aqueous solutions where the biological reactions take place. In this study, ligninase (LiP) from Phanerochaete chrysosporium was modified by poly(ethylene glycol) to enhance its activity and stability for the biodegradation of pentachlorophenol (PCP) in the presence of acetonitrile (MeCN), a water-miscible solvent. The modified enzyme retained 100% of its activity in aqueous solutions and showed enhanced tolerance against the organic solvent. The activity of the modified enzyme was found to be over twice that of the native enzyme in the presence of 10% (v/v) MeCN. The solubility of PCP was enhanced significantly by the addition of MeCN to aqueous solutions, such that it was over 10-fold more soluble in the presence of 15% (v/v) MeCN than in pure aqueous buffer solution (from 0.06 to 0.65 mM). Capitalizing on the enhanced substrate solubility and the increased activity of the modified enzyme, the catalytic efficiency of the modified LiP in solutions containing 15% MeCN was over 11-fold higher than that of the native enzyme in buffer solutions (pH 4.2) in unoptimized reactor systems (from 44 to 480 mol PCP/mol LiP.h). Continued research both in the use of organic solvents to increase the availability of recalcitrant contaminants and in the modification of enzymes to enhance their activity and stability in such solvents promises to dramatically affect our ability to remediate contaminated sites. Published by John Wiley & Sons.