Eosin, also known as eosin, is a peach or pink powder with the molecular formula C₂₀H₆O₅Br₄Na₂ and molecular weight 691.86. Eosin is most suitable for dyeing with hematoxylin to show the morphology and structure of normal or pathological tissues, and the water-soluble eosin contains a quinone-type benzene ring chromophore and two acidic chromophores that form sodium salts (R-COONa, R-ONa). When dissolved in water, the salt-forming acidic dye dissociates into the negatively charged chromic acid part (R-COO-, R-O-), i.e., the colored part of the dye, and the positively charged sodium ion part (Na+), and the negatively charged chromic acid part of the dye combines with the positively charged substances in the tissues in the form of ionic bonding to develop the color.

Eosin staining solution is simple to operate, does not use mercury, methanol and other toxic reagents, for the most commonly used concentration of water-soluble eosin staining solution, the reagent can be used for staining of tissue sections or cultured cells, often used in conjunction with Hematoxylin Staining Solution, but also used in conjunction with immunofluorescence staining or immunohistochemistry staining, which, on the one hand, can be used in the staining of eosin staining followed by immunofluorescence staining or staining of other dyes. On the other hand, it can also be immunohistochemical staining followed by eosin re-staining, the staining solution can be reused until it is considered ineffective and then replaced with a new staining solution, the stained cell plasma is pink or red.