Vat dyes are insoluble in water, but under the action of reducing agents, carbonyl groups are reduced to leuco acids with enol structures. The leuco acid is soluble in alkaline aqueous solutions and exhibit an affinity for cotton fibers, being adsorbed onto the fibers. as colorless bodies. Under the action of acid and oxidizing agents, they revert to their original insoluble carbonyl (quinoid or ketone) state, thus becoming fixed onto the fibers. Because a reducing agent is used in the dyeing process, they are called vat dyes.

When dyeing with vat dyes, sodium hydroxide is used as the alkali, and sodium hydrosulfite is used as the reducing agent; during printing, sodium carbonate and potassium carbonate are used as alkalis, and sodium hydrosulfite is replaced by sodium formaldehyde sulfoxylate as the reducing agent. Vat dyes have a complete range of colors, vibrant hues, and comprehensive fastness properties, especially excellent lightfastness. The production process of vat dyes is relatively complex, with many steps and operations, and in the past, a country's dye industry production level was measured by the production of vat dyes.

The dyeing of vat dyes can be divided into two methods based on the form of dyeing on the dye, namely, the colorless body dyeing method and the suspended body dyeing method. However, the latter requires the vat dye to be ground very finely to achieve a so-called ultrafine powder state. Some varieties of vat dyes, after pigmentization treatment, can also be used as organic pigments. Due to the large size of vat dye molecules (mostly polycyclic compounds) and their bright colors, as well as good heat resistance, vat dyes become important organic pigments after pigmentization treatment.

In practice, it has been found that although unreduced vat dyes have no affinity for cotton fibers, they do have affinity for polyester fibers. The dyeing reaction can be completed relying on high temperature, and post-dyeing reduction cleaning is required to remove excess dye. Therefore, some varieties of vat dyes themselves are also dispersing dye varieties.

Vat dyes are widely used in the dyeing and printing of cotton, viscose, and linen textiles, and some varieties of vat dyes can also be used for dyeing and printing synthetic fibers such as polyester, polyacrylonitrile, as well as polyester/cotton and polyester/viscose blended fabrics. They are also used for coloring coatings, paper, soaps, inks, plastics, paints, and rubbers. In the textile industry, vat dyes face challenges from coatings and reactive dyes, but they still occupy an important position in dyeing.