Production of extracellular esterase cleaving phenolic sidegroups from xylan was investigated using strains of Aspergillus. The activity levels produced were unrelated to the ferulic acid content of the lignocellulosic raw material in the cultivation medium. No correlation between the production of feruloyl esterase, acetyl xylan esterase and acetyl esterase activities was observed either. An esterase produced by Aspergillus oryzae was purified to electrophoretic homogeneity. The enzyme was an acidic monomeric protein having an isoelectric point of 3.6 and a molecular mass of 30 kDa. It was most active in the pH-range from 4.5 to 6.0 and was stable at temperatures up to 45°C. The esterase had a wide substrate specificity, liberating ferulic, p-coumaric and acetic acids from steam-extracted wheat straw fragments and acetic acid from acetylated xylo-oligomers and α-naphthyl acetate. The enzyme acted in synergism with other xylanolytic enzymes, which was reflected in increased production of phenolic acids from wheat straw xylo-oligosaccharides in the presence of xylanases of Trichoderma reesei.