Acid dyes are water soluble anionic dyes which are applied to fibres such as silk, wool, nylon & modified acrylic fibres from neutral to acid dyebaths. Attachment to the fibre is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fibre. Acid dyes are not substantive to cellulosic fibres.

Acid dyes sound scary to some novices, who imaging 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.

Acid dyes are classified into several classes: 1, leveling acid or strong acid dye, 2, super milling or fast acid or neutral acid dyes 3, milling or weak acid dyes, and. Leveling Acid Dyes are been sold under the trade names of Kiton, Erio, Intracid, Sandolan, and Amacid, among others; they are also the acid dye component of all-purpose or union dyes such as Rit and Tintex, says Knutson. It's difficult now to find out which specific acid dyes fall into which of these dye classes, however. At least part of the reason is that the information is not particularly useful to the dyer.