3,3′,5′-tetramethylbenzidine, or TMB is a white solid that forms a pale blue-green liquid in solution with ethyl acetate. TMB is degraded by sunlight and fluorescent lights. TMB can act as a hydrogen donor for the reduction of hydrogen peroxide to water by peroxidase enzymes such as horseradish peroxidase. The resulting one-electron oxidation product is a diamine-diamine complex, which causes the solution to take on a blue colour, and this colour change can be read on a spectrophotometer at the wavelengths of 370 and 650 nm. The reaction can be halted by the addition of acid or another stop reagent. Using sulfuric acid turns TMB yellow, with a peak absorbance of 450 nm. The amount of converted TMB may be indexed by the amount of 450 nm light it absorbs.