p-Coumaric acid (4-hydroxycinnamic) and ferulic acid (4-hydroxy-3-methoxycinnamic), two major hydroxycinnamic acids found in the complex cell walls of oat hulls, act as cross-linking agents between lignin and polysaccharides or between polysaccharides. As such, they are inhibitory to the biodegradation of cell walls by microorganisms. A previous study showed that Aspergillus feruloyl esterase with Trichoderma xylanase was able to break the ester linkage between ferulic acid and the attached sugar, releasing ferulic acid from the cell wall. The objective of this study was to investigate the specificity and the ability of Aspergillus feruloyl esterase to release p-coumaric acid from oat hulls. The results show there was no extensive release of p-coumaric acid in both the absence and presence of Trichoderma xylanase by Aspergillus feruloyl esterase. This indicates a specificity of Aspergillus feruloyl esterase, which is more active only on esters of certain hydroxycinnamic acids; in this case, Aspergillus feruloyl esterase will only sufficiently break the ester-linked feruloyl group but not the p-coumaroyl group in the complex cell walls of oat hulls. (C) 2004 Society of Chemical Industry.