Acid dyes are classified based on their chemical structure and dyeing conditions into categories such as strong acid dyes, weak acid dyes, acid mordant dyes, and acid complex dyes.

Strong Acid Dyes

These were the earliest developed acid dyes and require a strongly acidic dye bath. They feature simple molecular structures and low molecular weights, containing sulfonic or carboxylic acid groups. They have low affinity for wool but exhibit good leveling properties (migrating well on the fiber to produce uniform color), earning them the name "acid leveling dyes." However, they yield lighter shades, have poor wash fastness, can damage the wool during dyeing, and result in a harsher hand-feel. An example is Acid Red G (C.I. Acid Red 1).

Weak Acid Dyes

Weak acid dyes are derived from strong acid dyes by increasing molecular weight or introducing groups such as arylsulfonyl or long carbon chains. They possess more complex molecular structures and higher affinity for wool, allowing for dyeing in weakly acidic media without damaging the fiber. They produce deeper shades and offer improved fastness, though they lack leveling properties. An example is Weak Acid Brilliant Blue RAW (C.I. Acid Blue 80).

Acid Mordant Dyes

These are acid dyes that form metal complexes on the fabric after treatment with metal salts (such as chromium or copper salts) acting as mordants. Although the mordanting process is more complex, the resulting dyed products exhibit superior light, wash, and rub fastness. An example is Acid Mordant Black T (C.I. Mordant Black 11).

Acid Complex Dyes

These are formed by complexing certain acid dyes with metals such as chromium or cobalt. They are water-soluble, and the dyed products exhibit excellent light fastness. Its dye structure is similar to that of acid mordant dyes; however, a metal atom is incorporated into the azo dye molecule during synthesis—with a metal-to-dye ratio of 1:1—hence the designation "1:1 metal-complex dye." No additional mordanting treatment is required during the dyeing process. An example is Acid Complex Yellow GR (i.e., C.I. Acid Yellow 99).