Cellulose is the most abundant polymer on earth and plays a key role in the carbon cycle, agriculture, and human health. Many anaerobic cellulose-degrading bacteria produce uncharacterized yellow-orange, cellulose-binding pigments known as yellow affinity substances (here referred to as flavoaffinins) that are associated with efficient cellulose degradation. Here, we isolate and structurally characterize the flavoaffinins from