4-Amino-3-methylbenzenesulfonic acid, also known as 2-methyl-5-aminobenzenesulfonic acid, is an organic compound featuring both an amino group and a sulfonic acid group on a methyl-substituted benzene ring. This bifunctional structure makes it a versatile aromatic building block in synthetic chemistry. Its primary industrial application is as a key intermediate in the synthesis of azo dyes and pigments. The amino group allows for diazotization and subsequent coupling reactions to form the azo linkage, which is central to many colorants. The sulfonic acid group enhances the water solubility of the resulting dyes. It is specifically used in the production of acid dyes and direct dyes for textiles, as well as in the manufacture of certain organic pigments. Beyond its role in dye chemistry, this compound serves as a useful precursor for other sulfonated aromatic derivatives in research and fine chemical synthesis. It is a solid at room temperature and typically requires careful handling due to its potential to cause skin and eye irritation.