Graphene oxide flakes functionalized with 3-amino-1-propanesulfonic acid (denoted as GOSULF) as a powder or incorporated into an ionomer membrane such as Nafion (DuPont) were studied for water purification applications. The adsorption and the photocatalytic activity of the GOSULF powder itself or confined as a nano-additive in the membrane (Nafion–GOSULF) were investigated by measuring the degradation of a cationic dye, methylene blue (MB), in the dark and under UV/Visible light illumination. The results were compared with the ability of these systems to degrade an anionic dye, methyl orange (MO), in order to evaluate the role of the polymer–dye interaction. The degradation of the azo dyes depends on the mutual interaction between GOSULF flakes, the polymeric matrix and the dye: Nafion–GOSULF strongly reduces MB both under dark and illumination conditions in the same way as the GOSULF …