Acid Dyes are water soluble anionic Dyes that are applied to fibres such as nylon, wool, silk and modified acrylic fibres from neutral to acid dye baths. Attachment to the fibre is considered, at least partly, to salt formation between anionic groups in the Dyes and cationic groups in the fibre. Water soluable Acid Dyes are not substantive to cellulosic fibres. Acid Dyes are used both commercially and by the studio dyer to dye protein/animal fibers such as wool, silk, mohair, angora, alpaca and some nylons and synthetics. Acid Dyes makes the use of an acid such as vinegar, acetic or sulphuric acid to set the colour.

Acid Dyes does sound scary to some novices, who think that the Dyes themselves are caustic strong acids. In fact, the Dyes are non-caustic, are in many cases non-toxic, and are named for the mild acid (such as vinegar) used in the dyeing process, and for the types of bonds they form to the fiber. Some of them are significantly more toxic than fiber reactive Dyes, while others are even safe enough to eat, and are sold as food coloring agent.