The surface of polyacrylonitrile polymer (containing 10% acetate groups) as fibers and finely ground powder have been modified by enzymatic treatment. The enzyme used was a nitrile hydratase, member of the class of nitrile converting enzymes, present in the microorganisms Brevibacterium imperiale and Corynebacterium nitrilophilus. The pendant nitrile groups were selectively converted into the corresponding amides as assessed by XPS analysis. As indicated by the increase of the O/C atomic ratio, the fiber surface showed a significant increase in hydrophilicity. The newly formed amide groups were then able to react with the acid dyes typically used to stain natural fibers, conferring the coloring properties to the otherwise inert polymer surface.