Solvent Red 26, also known as C.I. 26120, is a purplish red synthetic azo dye. Its chemical formula is C₂₅H₂₂N₄O, or 1-,5-dimethyl-4-[(2-methylphenyl)azo]-phenyl]azo]-2-naphthol. It is soluble in oils and insoluble in water.

Its main use is as a standard fuel dye in the United States of America mandated by the IRS

Internal Revenue Service

The Internal Revenue Service is the revenue service of the United States federal government. The agency is a bureau of the Department of the Treasury, and is under the immediate direction of the Commissioner of Internal Revenue...

 to distinguish low-taxed or tax exempt heating oil

Heating oil

Heating oil, or oil heat, is a low viscosity, flammable liquid petroleum product used as a fuel for furnaces or boilers in buildings. Home heating oil is often abbreviated as HHO...

 from automotive diesel fuel, and by the EPA

United States Environmental Protection Agency

The U.S. Environmental Protection Agency is an agency of the federal government of the United States charged with protecting human health and the environment, by writing and enforcing regulations based on laws passed by Congress...

 to mark fuels with higher sulfur

Sulfur

Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...

 content; it is however increasingly replaced with Solvent Red 164

Solvent Red 164

Solvent Red 164, also called Oil Red B, is a synthetic red diazo dye. Its chemical structure is 1-Solvent Red 164, also called Oil Red B, is a synthetic red [[diazo dye]]. Its chemical structure is 1-...

, a similar dye with longer alkyl chains, which is better soluble in hydrocarbons. http://www.chevron.com/products/prodserv/fuels/bulletin/diesel/L2_3_9_rf.htm The concentration required by IRS is a spectral equivalent of 3.9 pounds per 1000 barrels, or 11.13 mg/l, of Solvent Red 26 in solid form; the concentrations required by EPA are roughly 5 times lower.
See also Solvent Red 27, Solvent Red 164

Solvent Red 164

Solvent Red 164, also called Oil Red B, is a synthetic red diazo dye. Its chemical structure is 1-Solvent Red 164, also called Oil Red B, is a synthetic red [[diazo dye]]. Its chemical structure is 1-...

.