Acid dyes are water-soluble anionic (negatively charged) dyes used primarily on protein fibers like wool and silk, as well as synthetic polyamide fibers like nylon. Chemically, they are typically sodium salts of organic acids, such as sulfonic or carboxylic acids. The application process requires an acidic dye bath, typically adjusted with acetic or formic acid. This acidity protonates the amine groups in the fiber, creating positive sites that form strong ionic bonds with the negatively charged dye molecules.

Acid dyes are celebrated for producing an exceptionally bright, vibrant color palette with excellent leveling properties and high lightfastness. However, their washfastness varies considerably depending on the specific molecular weight of the dye; heavier pre-metallized or milling acid dyes offer much better wet-fastness than simpler leveling acid dyes, which can bleed easily in warm water without chemical post-treatments.