The effect of hydroxycinnamic (ferulic, p-coumaric, sinapic and caffeic) and benzoic (benzoic, vanillic) acids on feruloyl esterase production by the filamentous fungus Aspergillus niger was assessed. A niger was grown on oat spelt xylan with and without the addition of hydroxycinnamic or benzoic acids. Only supplementation with ferulate or vanillate increased the production of feruloyl esterase, as measured by the hydrolysis of methyl ferulate in the culture supernatant. The other phenolic compounds reduced esterase levels over a 7 day period. In contrast, endo-β-1,4-xylanase levels were suppressed by the presence of ferulic, caffeic and sinapic acids and stimulated by p-coumaric, benzoic and vanillic acids.