Arabinoxylans (AXs) are the major dietary fibres in wheat and rye grains. They are resistant to digestion in the stomach and small intestine, therefore, they pass to the large intestine and are fermented by the colonic microbiota. The microbiota produce enzymes and degrade AX. Despite the proposed health benefits of AX fermentation, the mechanisms by which AX is fermented is still unknown. The degradation of AX requires the concerted action of at least three types of enzyme - endo-β-1,4-D-xylanase to cleave the backbone and exo-α-L-arabinofuranosidase/ exo-β-1,4-D-xylosidase to liberate arabinose/xylose which are subsequently catabolised. This study investigated the enzymatic metabolism of water-soluble wheat and rye AXs under in vitro fermentation conditions with a faecal inoculum from pigs fed a defined diet. By using enzyme assays and detailed structural analysis of the residual AX during fermentation, this study, for the first time, provides a model for the competitive-cooperative utilisation of AXs by the colonic microbiota.