A mordanting substrate is provided including a porous substrate, and, distributed within the substrate, a mordanting agent. Preferably, the substrate is a fibrous cellulosic material, and the mordanting agent is a cationic polymer that is soluble in polar solvents, preferably water-soluble. The polar solvent solubility of the polymer, in combination with the porosity and cellulosic nature of the material, has been found to cause the mordanting agent, when imbibed into the substrate in the form of a solution, to bind to fibers within the substrate. This binding in turn allows excellent immobilization of dye within the substrate.