An improved mordant solution and process for preparing fibers for dyeing and fixing natural dyes to fibers, particularly but not exclusively cellulose fibers, including cotton and linen, and synthetic fibers, including Rayon3 and Tensel . The improved natural mordant solution comprises an aqueous solution of alum KA1(SO4) and soda ash Na2CO3, most preferably an aqueous colloidal suspension of aluminum hydroxide. This mordant solution significantly improves the recognized measurable qualities of naturally dyed fibers, including light and washfastness, without using polluting heavy metal mordants. The improved mordant treatment and natural dye process includes first pretreating the fabric with an aqueous mordant wash, which increases the dye uptake in the dye bath, then treating the fabric with a second aqueous mordant wash, following dyeing, then preferably treating the dyed fabric with a final tannin wash, permanently setting the natural dye in the fibers or fabric.