A simple method for the direct catalytic heterogeneous modification of polysaccharides is presented. The novel method is exemplified by the combination of organic acid-catalyzed esterification and copper-catalyzed Huisgen reaction (click chemistry) to attach a fluorescent probe to solid cellulose. The heterogeneous 'organoclick' derivatization of cellulose allows for a mild, highly modular surface modification of cellulose under environmentally benign reaction conditions.