Uncharacterized side activities of enzymes can have significant negative effects on reaction products and yields. Hence, their identification and characterization is crucial for the development of successful reaction systems. Here, we report the presence of feruloyl esterase activity in CtXyn5A from Acetivibrio thermocellus besides its well-known arabinoxylanase activity for the first time. Both reaction types appear to be catalysed in the same active site in two subsequential steps. The ferulic acid substituent is cleaved off first, followed by the hydrolysis of the xylan backbone. The esterase activity on complex carbohydrates was found to be higher than the one of a designated ferulic acid esterase (E-FAERU). Therefore, we conclude that the enzyme exhibits a dual function rather than an esterase side activity.