Natural fibers have gained much attention as reinforcing components in composite materials. Despite several interesting characteristics like low cost, low density, high specific properties and biodegradability they show poor compatibility with the polymer matrix. We have shown that it is possible to use a laccase from Trametes hirsuta as a biocatalyst to attach different types of functional phenolic molecules onto the fibers. A 5% incorporation of the functional molecules was achieved as measured via X-ray photoelectron spectroscopy (XPS) in flax although it was lower in coconut fibers. In combination with different mediators it was possible to broaden the activation scope and graft hydrophobic molecules like dimer fatty amines. Among the different mediators tested 1-hydroxybenzotriazole (HBT), 2,2,6,6-tetramethylpiperidin-1-yloxy (TEMPO) and 2'-azino-bis(3-ethylbenzothiazoline-6-sulphonic acid) (ABTS), TEMPO were the most effective achieving a 10% increase in carbon as measured by XPS.